BAIRD &TATLOCK (LONDON) LTD. 



Light and shade instruments are not, however, sufficiently sensitive for practical purposes, and of recent years a great impetus 

 has been given to polarimetry by the introduction of the half-shadow instruments. In these the mechanism for sensitiveness 

 is arranged so as to produce a field divided into two halves, which in certain positions of the analyser are unequally illuminated, 

 but in one position capable of being fixed with great accuracy they exhibit a uniformly faint shade. This position is taken 

 as the zero or point of reference. The half shade is brought about by using as a polariser a half-shade prism of particular construc- 

 tion, or by a semi-disc of quartz as in the Laurent instrument. 



SPECIFIC ROTATORY POWER. 



To determine the specific rotatory power of an active substance it is necessary to know 

 a, The observed angle of rotation in angular degrees. 



c. The concentration of the solution in grammes per loo cubic centimetres. 

 L, The length of the column of the solution in millimetres. 

 The specific rotation [a] can be expressed by the formula 

 10*. a 



[a]= 



L. c 



The value of a varies with the refrangibility of the polarised ray, and it is necessary to state the position in the spectrum of 

 the particular ray employed. There are two systems of expressing the rotation of a substance in general use. In the one it is 

 referred to the D line of the solar spectrum, the rotation being then expressed by [a] D ; in the other it is referred to the " medium 

 yellow ray " (jaune moyen), the colour complementary to Biot's " transition tint," in which case the specific rotation is expressed 

 by [a]j. For the determinations of [a]o the sodium light is employed with aWild, Mitscherlich, Jellet-Cornu, or Laurent instrument, 

 and the direct rotation is found in degrees of arc. The determinations of [alj are made with neutral tint or half-shade polari- 

 meters, such as the Ventzke-Scheibler. The scale divisions in these instruments are arbitary, and require to be converted into 

 angular degrees before the specific rotation can be calculated. Since the readings obtained with such instruments are based on 

 the rotation of a quartz plate, they must of necessity be in terms of the rotation of a quartz plate of definite thickness. Attention 

 may be here called to the fact that the rotatory dispersions of quartz and cane sugar solutions are practically identical. The 

 quartz compensating instrument it will be seen, cannot be used for the comparison of the rotation of substances, having a different 

 power of rotatory dispersion to that of quartz, unless the relation of the rotatory dispersion of the substance under examination 

 to that of quartz is known. 



The wave length of this mean yellow light is less than that of the ray D, which lies on the border between orange and yellow, 

 so that the numerical value of [n]j is always greater than that of [a] D . Broch has found with a quartz plate I millimetre thick 

 that [a] D = 21.67 degrees and [aj = 24.5 degrees, so that to express one in terms of the other 



24-5 9 



[ajj = [a] D = 1.1306 [a] D or approximately = -[a] D 



21.67 8 



21.67 8 



[a]o tz [a]j = 0.8845 [a]j or approximately -[a]j 



24.5 9 



The proportion between [a]j and [a] D varies in different substances according to their different rotatory dispersions. 



Thus for quartz .. .. .. .. .. .. .. [a] D : [a]j =1:1.131 



solutions of sugar .. .. .. .. .. .. .. [a] D : [ajj =1:1.129 



alcholic solutions of camphor .. .. .. .. .. .. 1:1.198 



oil of turpentine . . . . . . . . . . . . . . . . I : 1.243 



PRECAUTIONS TO BE OBSERVED IN USING THE POLARISCOPE. 



Before filling the tube it must either be thoroughly dried by pushing a plug of filter paper or some other soft material through 

 it, or it should 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 by 

 the screw or spring cap and cover glass, and grasped by the other end with the thumb and finger. The solution is poured into 

 it until the meniscus projects slightly above the opening, the air bubbles allowed time to rise, and the cover glass pushed hori- 

 zontally over the end of 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. The cover glass being 

 in position, the tube is closed by screwing or clamping on the cap as the case may be. The greatest care must be taken in screw- 

 ing down the caps that they do not press too tightly on 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. 



Before taking the actual reading, observations are made of the zero and with a standard quartz plate of known rotation. 

 The mean of several readings is taken and corrected for any deviation of the zero. 



In the polarisation of the quartz plates and colourless solutions, difficulty may be experienced on obtaining a complete 

 correspondence of both halves of the field. This may be overcome and the neutral point found, but when it cannot, the ordinary 

 I eyepiece of the instrument may be replaced by another which is supplied with the polariscope, and which carries a section of a 

 crystal of bichromate of potassium. This removes the difficulty and renders it possible to obtain a field of exact neutrality. 



The instrument claiming most attention at present, and which on account of its increased accuracy over the half-shadow 

 ; arrangement has a great future before it, is the one in which the optical field is divided into three parts. It gives the greatest satis- 

 faction to those who use it, and the zero point remains constant after many months of continual usage. 



For those who intend purchasing an instrument for general sugar work, or, in fact, for all work in which substances are 

 examined which have a rotatory dispersive power equivalent to that of quartz, we strongly recommend this new polariscope. 



Messrs. Schmidt & Haensch have also made certain improvements in instruments which are to be used in tropical climates, 

 i whereby the prisms are protected from excessive weathering. With ordinary care, however, any polariscope should be able to 

 withstand a tropical climate. 



The new form of tube which allows the presence of a small bubble without interfering with the reading, will be found very 

 useful for volatile liquids. It is a decided improvement on the tube which had an enlargement in the centre. 



The modified form of polarimeter described by H. Landolt (Ber., 1895, 28, 3102-3104) is admirably adapted for scientific 

 purposes. Its construction is similar in principle to the instrument described in 4230 ; it is, however, much cheaper. It also 

 .facilitates the examination of rotatory liquids at temperatures extending over a wide range. 



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CHEMICAL AND SCIENTIFIC APPARATUS AND PURE CHEMICALS. 



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