,,. 



BY DR. LAJDER BRUNTON. 571 



cub. cent. 100 cub. cent, contain T T th of an equivalent in grammes (4 

 grammes) of hydrate of soda, and will neutralize ^th of an equivalent 

 in grammes of an acid. 



Soda solution for estimating the acidity of gastric juice is made by di- 

 luting 100 c. c. of the above solution to the bulk of a litre. 



218. Polariscope. There are several organic substances whose 

 solutions possess the power of circumpolarization, i. e., of rotating to one 

 side or another the plane of polarization of a ray of polarized light passing 

 through them. Some of them, such as glucose, cane sugar, and tartaric 

 acid, turn it to the right hand, while others, such as albumin, unc-rystal- 

 lizable sugar, and oil of turpentine, turn it to the left. As the amount of 

 rotation increases in proportion to the concentration of the solution and 

 the thickness of the stratum through which the ray passes, it is easy to 

 ascertain the quantity of a substance held in solution by simply observing 

 the extent to which a ray is rotated in passing through a stratum of a de- 

 finite thickness. The apparatus used for this purpose is shown in rig. 853. 

 It consists of a stand in which arc placed two NicoPs prisms, a and b. 

 The prism b is fixed, but that at a is movable, and the extent to which 

 it is rotated is indicated on a graduated circular disk s s by an index z. 

 When the two prisms are placed exactly in the same position, the ray, 

 which has been polarized by b passes readily through , and the field of 

 vision of an observer, looking into the instrument at a, is illuminated. 

 As a is turned round on its axis, the field becomes dimmer and dimmer 

 till the tAvo prisms are turned crosswise to each other, when the polar- 

 ized ray by b is entirely stopped by , and the field consequently be- 

 comes quite dark. At this time the index stands at zero. If a glass 

 tube, containing a solution of sugar or albumin, is then placed in the 

 space oo, the polarized ray will pass through it, and in doing so will 

 have its plane of polarization more or less rotated, so that it will no 

 longer be entirely stopped by the prism a. In order, therefore, to stop 

 it again and produce a dark field, this prism must be rotated to a corre- 

 sponding degree, and the extent of rotation is read off on the graduated 

 disk. As it is difficult to determine exactly the position of a. at which 

 the field is darkest, some additions have been made to this instrument 

 by Soleil and Ventzke, which make their saccharimeter more complica- 

 ted, but greatly increase its exactitude. The first of these is a plate of 

 quartz, q, composed of two pieces, whose line of junction is exactly in 

 the middle of the field of vision. One piece rotates light to the right 

 hand, while the other turns it to the left. When a solution of sugar 

 is placed in the space o o, it increases the action of that half of the plate 

 which rotates to the right, and lessens the action of the other half which 

 rotates to the left, and the two halves of the field of vision become of a 

 different color. This difference can be removed by turning the prism 

 , but this is more easily effected by means of the compensator n. The 

 chief parts of this are figured separately. It consists of two equal 

 prisms (> and r') of left-handed quartz, whose surfaces (c and O an? 

 cut perpendicularly to the optic axis of the crystal. Taken together 

 they form a plate bounded by parallel surfaces, and they can be made 

 to slide on one another by means of a rack and pinion, 9, so as to in- 

 crease or diminish its thickness at will. One of the frames in which 

 these is fixed has a scale, I, and the other a vernier, n. When the zero 

 of this corresponds to the zero on the scale, the left-handed rotation of 

 the two prisms is compensated by a plate of right-handed quartz, p. and 

 the field then appears of an uniform color, but as soon as the prisms are 

 moved this compensation ceases, and the two halves become differently 

 colored. The same effect is produced by putting a solution of sugar 

 into o o. The screw v is then turned till the effect of the sugar is counter- 

 balanced and the amount of rotation read off on the scale. At this end, 



