CHAP. I.] 



THE PROTEIDS. 



In all experiments on dialysis care has to be taken, before an experi- 

 ment is commenced, to ascertain that the parchment-paper is quite free 



Fig. l. 



HOOP DIALYSER. Fig. 2. BULB DIALYSER. 



DlALYSERS DESCRIBED IN THE TEXT. 



Proteids all 

 rotate the 

 plane of po- 

 larized light 

 to the left. 

 Determina- 

 tion of their 

 Specific Ro- 



from even the minutest holes. These are readily detected if the outer 

 surface of the dialyser (i. e. the surface which during the actual experiment 

 is to be immersed in water) be dried and placed upon a sheet of filtering 

 paper, and then water poured into the interior ; a leak being evidenced 

 by the appearance of moisture on the outside. 



Amongst the organic constituents of the animal 

 body a large number when dissolved possess the power 

 of rotating the plane of polarized light ; as for instance 

 the proteids, the sugars, the bile acids, &c. The deter- 

 mination of the fact that a solution of a body rotates 

 the plane of polarized light in a definite direction and 

 to a definite extent is sometimes of great service in 



tatory power, aiding its identification, and in enabling its amount 



to be determined. 



As the rotation exerted by an active body dissolved in an inactive 

 liquid is dependent upon the molecules of the active body existing in 

 solution, the degree of rotation will in the case of any particular substance 

 be proportional to the number of active molecules traversed by the light, 

 and therefore proportional to the length of the column of liquid traversed, 

 and to the degree of concentration, of the solution. If, for instance, a 

 column of solution of any active substance, say of cane-sugar, of any given 

 length, rotate the plane of polarized light x degrees, then if the column be 

 doubled the rotation will amount to 2x degrees; or the double rotation will 

 be observed if instead of doubling the length of the tube the amount of 

 active substance in a given volume of liquid be exactly doubled. It can 

 be shewn that any active body rotates to different degrees the plane of 

 polarization of light of different colours. In determining, therefore, the 



