1879.] 



Researches on Lactin. 



273 



III. "Researches on Lactin." By Edmund J. Mills, D.Sc, 

 F.R.S., " Young" Professor of Technical Chemistry in An- 

 derson's College, Glasgow, and James Hogarth. Received 

 December 4, 1878. 



Although lactin, or sugar of milk, has been investigated by nume- 

 rous chemists, there are many problems connected with it which still 

 await solution. We have accordingly undertaken a series of experi- 

 ments in connexion with this remarkable compound, in the hope, not 

 only of obtaining special results, but such as may be made available 

 in studies of a more general nature. As our work throughout has 

 been for the most part optical as well as chemical, we have first to 

 state our methods of obtaining the constant of Jellett's polarimeter, 

 the instrument employed in our investigations. 



I. Determination of the Polarimeter's Constant. — a. By quinine sul- 

 phate. 5*5412 grms. of the sulphate were dissolved in water acidu- 

 lated with hydric sulphate, and the solution made up to 100 cub. 

 centims. The average of five readings gave a solution of —25°" 73, 

 equivalent to a specific rotatory power of — 232°*16. De Gris and 

 Alluard* give — 255°'6, a number which is to our experimental num- 

 ber as 1-10096 to 1. 



(3. By cane sugar. Three sets of experiments on solutions contain- 

 ing respectively 16*3500, 8*1750, and 4*0875 grms. in 100 cub. centims., 

 and embracing five, four, and four readings, gave a general mean 

 reading 21°* 74, equivalent to a specific rotation 66°*48. 



This is to the geuerally accepted number (73°*8) as 1 to 1*11011. 



7. By salicin. Two sets of experiments with solutions containing 

 respectively 4*9156 and 2*4578 grms. in 100 cub. centims., and each 

 embracing three readings, gave a general mean reading 4°*92, equal to 

 a specific rotation 50°*046. Bouchardatf gives 55°*832, which is to 

 the number got by Jellett's instrument as 1*11561 to 1. 



The average of the three numbers, 1*10096, 1*11011, and 1*11561, 

 gives 1*10889 as an experimental factor for converting our Jellett 

 readings into ordinary readings. 



The relation of the two scales may also be seen by examining the 

 arc divided to read percentages of cane sugar with a solution contain- 

 ing 16*35 grms. in 100 cub. centims. In the Jellett instrument, an 

 arc of 21°*666 is divided into hundredths for this purpose ; and as 

 16*35 grms. pure cannose read 100 on this scale, the specific rotation 



* " Compt. rend.," lix, 201. 

 t " Compt. rend.," xyiii, 298. 



