Chapter 9

The three different appearances presented by the field are best shown in the above diagram. With the milled head set at the point which gives the appearance of the middle disk as shown, the eye of the observer is raised to the reading tube, K, and the position of the scale is noted. It will be seen that the scale proper is attached to the quartz wedge, which is moved by the milled head, and attached to the other quartz wedge is a small scale called a vernier which is fixed, and which serves for the exact determination of the movable scale with reference to it. On each side of the zero line of the vernier a space corresponding to nine divisions of the movable scale is divided into ten equal parts. By this device the fractional part of a degree indicated by the position of the zero line is ascertained in tenths; it is only necessary to count from zero, until a line is found which makes a continuous line with one on the movable scale.

With the neutral field as indicated above, the zero of the movable scale should correspond closely with the zero of the vernier unless the zero point is out of adjustment.

If the observer desires to secure an exact adjustment of the zero of the scale, or in any case if the latter deviates more than one-half of a degree, the zero lines are made to coincide by moving the milled head and securing a neutral field at this point by means of the small key which comes with the instrument, and which fits into a nipple on the left hand side of F, the fixed quartz wedge of the compensating system. This nipple must not be confounded with a similar nipple on the right hand side of the analyzing prism, H, which it fits as well, but which must never be touched, as the adjustment of the instrument would be seriously disturbed by moving it. With the key on the proper nipple it is turned one way or the other until the field is neutral. Unless the deviation of the zero be greater than 0.5°, it will not be necessary to use the key, but only to note the amount of the deviation, and for this purpose the observer must not be content with a single setting, but must perform the operation five or six times, and take the mean of these different readings. If one or more of the readings show a deviation of more than 0.3° from the general average, they should be rejected as incorrect. Between each observation the eye should be allowed 10 to 20 seconds of rest.

The "setting" of the zero having been performed as above, the determination of the accurate adjustment of the instrument by means of the "control" quartz plates is proceeded with. Three such plates will be furnished with each polariscope, which have "sugar values" respectively approximating 80°, 90°, and 100°. These values may vary with the temperature, and tables are furnished with them which give their exact value at different temperatures, from 10° to 35° C.

One of these plates is placed in the instrument, and the field observed; it will be seen that the uniform appearance of the field is changed. The milled head is turned to the right until the exact point of neutrality is re-established, just as described above in setting the zero. The scale is read, the observation repeated, the reading taken again, and so on until five or six readings have been made. The average is taken, readings being rejected which show a divergence of more than 0.3, and the result corrected for the deviation of the zero point, if any was found, the deviation being added if it was to the left, and subtracted if to the right. If the adjustment of the instrument be correct, the result should be the value of the control plate used, as ascertained from the table, for the temperature of 20°. Each of the three plates is read in the instrument in this way. A variation of 0.3 from the established values may be allowed for errors of observation, temperature, etc., but in the hands of a careful observer a deviation greater than this with one of the three plates, after a careful setting of the zero, shows that the instrument is not accurately adjusted.

The complete verification of the accurate adjustment of the polariscope by means of three control plates, as given above, should be employed whenever it is set up for the first time by the officer using it, whenever it has sustained any serious shock or injury, and whenever it has been transported from one place to another. It should also be done at least once a week while the instrument is in active use.

After the complete verification has been performed as described, further checking of the instrument is done by means of one control plate alone, the one approximating 90°, and the setting of the zero point is dispensed with, the indication of the scale for sugar solutions being corrected by the amount of deviation shown in the reading of the 90° control plate from its established value as ascertained from the table, at the temperature of the room.

For example: A sugar solution polarizes 80.5; the control plate just before had given a polarization of 91.4, the temperature of the room during both observations being 25° C. According to the table the value of the control plate at 25° C. is 91.7; the reading is, therefore, 0.3 too low, and 0.3 is added to the reading of the sugar solution, making the corrected result 80.8. The temperature of the room should be ascertained from a standardized thermometer placed close to the instrument and in such a position as to be subject to the same conditions.

PREPARATION OF THE SUGAR SOLUTION FOR POLARIZATION.

If the sample is not entirely uniform it must be thoroughly mixed before weighing out, after all the lumps are broken up, best with a mortar and pestle. Then 26.048 grammes are weighed out on the balance in the tared German silver dish furnished for this purpose. Care must be taken that the operations of mixing and weighing out are not unduly prolonged, otherwise the sample may easily suffer considerable loss of moisture, especially in a warm room. The portion of sugar weighed out is washed by means of a jet from a wash bottle into a 100 c.c. flask, the dish being well rinsed three or four times and the rinsings added to the contents of the flask. The water used must be either distilled water or clear water which has been found to have no optical activity. After the dish has been thoroughly rinsed, enough water is added to bring the contents of the flask to about 80 c.c. and it is gently rotated until all the sugar has dissolved. The flask should be held by the neck with the thumb and finger, and the bulb not handled during this operation. Care must be taken that no particle of the sugar or solution is lost. To determine if all the sugar is dissolved, the flask is held above the level of the eye, in which position any undissolved crystals can be easily seen at the bottom. The character of the solution is now observed. If it be colorless or of a very light straw color, and not opalescent, so that it will give a clear transparent liquid on filtration through paper, the volume is made up directly with water to the 100 c.c. mark on the flask. Most sugar solutions, however, will require the addition of a clarifying or decolorizing agent in order to render them sufficiently clear and colorless to polarize. In such case, before making up to the mark, a saturated solution of subacetate of lead is added.

The quantity of this agent required will vary according to the quality of the sugar; for sugar which has been grained in the strike pan and washed in the centrifugals, from 3 to 15 drops will be required; for sugar grained in the strike pan but not well washed in the centrifugals, that is, sugar intended for refining purposes, from 15 to 30 drops will be required; for sugar not grained in the strike pan, that is, "wagon" or "string sugar," "second sugar," etc., from 1 to 3 c.c. will be required. After adding the solution of subacetate of lead the flask must be gently shaken, so as to mix it with the sugar solution. If the proper amount has been added, the precipitate will usually subside rapidly, but if not, the operator may judge of the completeness of the precipitation by holding the flask above the level of the eye and allowing an additional drop of subacetate of lead to flow down the side of the flask into the solution; if this drop leaves a clear track along the glass through the solution it indicates that the precipitation is complete; if, on the other hand, all traces of the drop are lost on entering the solution, it indicates that an additional small quantity of the subacetate of lead is required. The operator must learn by experience the point where the addition should cease; a decided excess of subacetate of lead solution should never be used.

The use of subacetate of lead should, in all cases, be followed by the addition of "alumina cream" (aluminic hydrate suspended in water)[2] in about double the volume of the subacetate solution used, for the purpose of completing the clarification, precipitating excess of lead, and facilitating filtration. In many cases of high grade sugars, especially beet sugars, the use of alumina alone will be sufficient for clarification without the previous addition of subacetate of lead.

[Footnote 2: Prepared as follows: Shake up powdered commercial alum with water at ordinary temperature until a saturated solution is obtained. Set aside a little of the solution, and to the residue add ammonia, little by little, stirring between additions, until the mixture is alkaline to litmus paper. Then drop in additions of the portion left aside, until the mixture is just acid to litmus paper. By this procedure a cream of aluminum hydroxide is obtained suspended in a solution of ammonium sulphate, the presence of which is not at all detrimental for sugar work when added after subacetate of lead, the ammonium sulphate precipitating whatever excess of lead may be present.]

The solution is now made up to the mark by the addition of distilled water in the following manner. The flask, grasped by the neck between the thumb and finger, is held before the operator in an upright position, so that the mark is at the level of the eye, and distilled water is added drop by drop from a siphon bottle or wash bottle, until the lowest point of the curve or meniscus formed by the surface of the liquid just touches the mark. If bubbles hinder the operation, they may be broken up by adding a single drop of ether, or a spray from an ether atomizer, before making up to the mark. The mouth of the flask is now tightly closed with the thumb, and the contents of the flask are thoroughly mixed by turning and shaking. The entire solution is now poured upon the filter, using for this purpose a funnel large enough to contain all the 100 c.c. at once, and a watch glass is placed over the funnel during filtration to prevent a concentration of the solution by evaporation.

The funnel and vessel used to receive the filtrate must be perfectly dry. The first portion of the filtrate, about 20 to 30 c.c., should be rejected entirely, as its concentration may be affected by a previous hygroscopic moisture content of the filter paper. It may also be necessary to return subsequent portions to the filter until the liquid passes through perfectly clear.

If a satisfactory clarification has not been obtained, the entire operation must be repeated, since only with solutions that are entirely clear and bright can accurate polarimetric observations be made.

When a sufficient quantity of the clear liquid has passed through the filter, the 200 mm. observation tube is filled with it. The 100 mm. tube should never be used except in rare cases, when notwithstanding all the means used to effect the proper decolorization of the solution, it is still too dark to polarize in the 200 mm. tube. In such cases the shorter tube may be used, and its reading multiplied by two. The zero deviation must then be determined and applied to the product. This will give the reading which would have been obtained if a 200 mm. tube could have been used, and it only remains to apply the correction determined by the use of the control plate as previously described.

Example:

Solution reads in 100 mm. tube 47.0 Multiplied by 2 2.0 ---- Product 94.0 Zero reads plus 0.3 0.3 ---- Solution would read in 200 mm. tube 93.7

Reading of control plate 90.4 Sugar value of control plate 90.5 ---- Instrument too low by 0.1 Add 0.1 to 93.7 ---- Correct polarization of solution 93.8

Before filling the tube it must either be thoroughly dried by pushing a plug of filter paper through it, or it must be rinsed several times with the solution itself. The cover glasses must also be clean and dry, and without serious defects or scratches. Unnecessary warming of the tube by the hand during filling should be avoided; it is closed at one end with the screw cap and cover glass, and grasped by the other end with the thumb and finger. The solution is poured into it until its curved surface projects slightly above the opening, the air bubbles allowed time to rise, and the cover glass pushed horizontally over the end of the tube in such a manner that the excess of liquid is carried over the side, leaving the cover glass exactly closing the tube with no air bubbles beneath it, and with no portion of the liquid upon its upper surface. If this result is not attained, the operation must be repeated, the cover glass being rubbed clean and dry, and the solution again brought up over the end by adding a few more drops. The cover glass being in position, the tube is closed by screwing on the cap. The greatest care must be observed in screwing down the caps that they do not press too tightly upon the cover glasses; by such pressure the glasses themselves may become optically active, and cause erroneous readings when placed in the instrument. It should therefore be ascertained that the rubber washers are in position over the cover glasses, and the caps should be screwed on lightly. It must also be remembered that a cover glass, once compressed, may part with its acquired optical activity very slowly, and some time must be allowed to elapse before it is used again.

The polariscopic reading may now be taken, an observation on the 90° control plate having been made immediately before as previously described. Then without altering the position of the instrument relative to the light, or changing the character of the latter in any way, the tube filled with the sugar solution is substituted for the control plate. The telescope is adjusted, if necessary, so as to give a sharply defined field, which must appear round and clear. (This condition must be fulfilled before the observation is performed, as it is essential to accuracy.) The milled head is turned until the neutral point is found, and the reading is taken exactly as previously described, the operation repeated five or six times, the average taken with the rejection of aberrant readings, the average figure corrected for the deviation shown by the control observation from the sugar value of the control plate at the temperature of observation as given in the table, and the result taken as the polarization of the sugar. When a series of successive polarizations is made under the same conditions as regards temperature, position of the instrument with relation to the high intensity, of the light, etc., the control observation need not be made before each polarization, one such observation being sufficient for the entire series. The control must be repeated at least once an hour, however, and oftener when the operator has reason to think that any of the factors indicated above have been altered, for any such alteration of conditions may change the zero point of the instrument.

In the polarization of the quartz plates, as also in the polarization of very white sugars, difficulty may be experienced in obtaining a complete correspondence of both halves of the field. With a little practice this may be overcome and the neutral point found, but when it cannot, the ordinary telescope of the instrument may be replaced by another, which is furnished with the polariscope and which carries a yellow plate. This removes the difficulty and renders it possible, even for one not well accustomed to the instrument, to set it at the exact point of neutrality.

SUMMARIZED SOURCES OF ERROR.

The following principal sources of error must be especially guarded against:

1. Drying out of sample during weighing.

2. Excess of subacetate of lead solution in clarification.

3. Incomplete mixing of solution after making up to mark.

4. Imperfect clarification or filtration.

5. Concentration of solution by evaporation during filtration.

6. Undue compression of the cover glass.

7. Alteration of the temperature of room, position of instrument, or intensity of light while the observation or control observation is being performed.

8. Performances of polarization with a cloudy, dim, or not completely round or sharply defined field.

In closing this report the members of this commission hereby signify their intention to promote uniformity and accuracy by adopting and using the standards and general plan of procedure recommended in this report in the polarimetric determinations over which, in their respective branches of government work, they have control.

* * * * *

THE GRAND FALLS OF LABRADOR.

Hamilton Inlet, or Ionektoke, as the Esquimaus call it, is the outlet to the largest river on the Labrador Peninsula, and of great importance to commerce, Rigolet, the headquarters of the Hudson Bay Company in this region, being situated on its shores. This inlet is the great waterway to Central Labrador, extending into the interior for nearly 200 miles.

This immense basin is undoubtedly of glacial origin, evidences of ice erosion being plainly seen. It is divided into two general basins, connected by the "narrows," a small strait, through which the water rushes with frightful rapidity at each tide. Into the head of the inlet flows the Hamilton, or Grand River, an exploration of which, though attended with the greatest danger and privation, has enticed many men to these barren shores. Perhaps the most successful expedition thus far was that of Mr. Holme, an Englishman, who, in the summer of 1888, went as far as Lake Waminikapon, where, by failure of his provisions, he was obliged to turn back, leaving the main object of the trip, the discovery of the Grand Falls, wholly unaccomplished.

It has been left for Bowdoin College to accomplish the work left undone by Mr. Holme, to do honor to herself and her country by not only discovering, measuring, and photographing the falls, but making known the general features of the inland plateau, the geological structure of the continent, and the course of the river.

On Sunday, July 26, a party of the Bowdoin expedition, consisting of Messrs. Cary, Cole, Young, and Smith, equipped with two Rushton boats and a complement of provisions and instruments, left the schooner at the head of the inlet for a five weeks' trip into the interior, the ultimate object being the discovery of the Grand Falls. The mouth of the river, which is about one mile wide, is blockaded by immense sand bars, which have been laid down gradually by the erosive power of the river. These bars extend far out into Goose Bay, at the head of Lake Melville, and it is impossible to approach the shores except in a small boat. Twenty-five miles up the river are the first falls, a descent of the water of twenty-five feet, forming a beautiful sight. Here a cache of provisions was made, large enough to carry the party back to the appointed meeting place at Northwest River. The carry around the first falls is about one and a half miles in length, and very difficult on account of the steep sides of the river.

From the first falls to Gull Island Lake, forty miles above, the river is alternately quick and dead water. Part of it is very heavy rapids, over which it was necessary to track, and in some places to double the crews. Each boat had a tow line of fifty feet, and in tracking the end was taken ashore by one of the crew of two, while the boat was kept off the bank by the other man with an oar. At the Horseshoe Rapids, ten miles above Gull Island Lake, an accident happened which threatened to put a stop to further progress of the expedition. While tracking around a steep point in crossing these rapids the boat which Messrs. Cary and Smith were tracking was overturned, dumping barometer, shotgun, and ax into the river, together with nearly one-half the total amount of provisions. In the swift water of the rapids all these things were irrevocably lost, a very serious loss at this stage in the expedition. On this day so great was the force of the water that only one mile was made, and that only with the greatest difficulty.

Just above the mouth of the Nimpa River, which enters the Grand River twenty-five miles above Gull Island Lake, a second cache of provisions was made, holding enough to carry the party to their first cache at the first falls. One of the boats was now found to be leaking badly, and a stop was made to pitch the cracks and repair her, making necessary the loss of a few hours. From Nimpa River to the Mouni Rapids, at the entrance to Lake Waminikapon, the water was found to be fairly smooth, and good progress was made. The change in the scenery, too, is noticeable, becoming more magnificent and grand. The mountains, which are bolder and more barren, approach much nearer to each other on each side of the river, and at the base of these grim sentinels the river flows silvery and silently. The Mouni Rapids, through which the water passes from Lake Waminikapon, presented the next obstacle to further progress, but the swift water here was soon passed, and well repaid the traveler with the sight here presented almost unexpectedly to his view.

The lake was entered about 4 o'clock in the afternoon, and, as the narrow entrance was passed, the sun poured its full rich light on rocky mountains stretching as far away as the eye could reach, on each side of the lake, and terminating in rocky cliffs from 600 to 800 feet in perpendicular height, which formed the shores or confines of the lake. Across Lake Waminikapon, which is, more properly speaking, not a lake at all, but rather a widening of the river bed, the progress was very good, the water having no motion to retard the boats, and forty miles were made during the day.

Here a misfortune, which had been threatening for several days, came upon the party. Mr. Young's arm was so swollen, from the shoulder to finger tips, that he could scarcely move it, the pain being excessive. It had been brought on doubtless by cold and exposure. Seeing that he could be of no further use to the party, it was decided to divide forces, Mr. Smith returning with the sick man to Rigolet for medical assistance. The separation took place August 8, when the party had been on the river eleven days. The party were very sorry to return at this point, since from the best information which they could get in regard to the distance, the falls were but fifty miles above them. Under the circumstances, however, there was no help for it. So Smith and Young, bidding their friends good fortune, started on their return trip. The mouth of the river was reached in three days, a little less than one-third the time consumed in going up, and that, too, with only one man to handle the boat.