1879.] 



Researches on Lactin. 



275 



were made at intervals of 2 hours, the results being given under 

 Table I, No. 21. 



See Table I. 



In order to increase the total change and lessen proportionally the 

 error of experiment, it became necessary to use a stronger solution, to 

 increase the length of column, and to reduce the interval elapsing 

 between first contact and first observation as far as possible. To 

 attain these conditions the following method was adopted : — About 

 10 grms. of powdered lactin were rubbed in a mortar with about 

 60 cub. centims. of water for half an hour, the solution filtered, and 

 the first observation taken one hour after first contact. The metal 

 tube belonging to the polariscope was also discarded, and a glass one 

 constructed from a piece of tubing 17 millims. wide, by sealing on a side 

 piece for the introduction of a thermometer, and grinding the ends 

 carefully until it measured 242 millims., the greatest length admitted 

 by the polarimeter. Two glass disks were cemented on the ex- 

 tremities, and the tube covered from end to end by a helix of thin tin 

 tubing, through which a current of water might be passed to keep the 

 temperature constant ; to guard further from variations in temperature 

 the tube was covered with cotton wadding. With these precautions 

 three experiments were made (Table I, Isos. 1, 2,3), the result being 

 that the total change was nearly doubled. In all the other experi- 

 ments the method was slightly varied, the lactin being placed in a 

 bottle with a ground glass stopper, 60 cub. centims. water placed on 

 it, and the whole shaken vigorously at intervals for half an hour, 

 filtered, and the first observation taken as before. Each experi- 

 ment extended over six hours, and included ten observations. For 

 each observation three or four readings were made, and the average 

 taken. In Nos. 4, 5, 6, 7, 8, 9, 13, 14, 15, 16, 17, 18, 19, 20, of the 

 accompanying table, varying weights of sodic and potassic chloride 

 were introduced. In every experiment the thermometer was read at 

 the same time with the rotation; and the average temperature, as 

 well as its extreme variation, is given in the table. That the different 

 experiments might be compared, we have expressed them by the 

 equation — 



y = a + bx -f- car 2 , 



in which y is the angle of rotation, x the time in half -hours, counting 

 from the first contact of the lactin with water, and a, b, and c, are 

 constants. The values of a, b, and c, were calculated by the method 

 of least squares. 



In Table II are given the equations, accompanied by the probable 

 error of a single comparison of the calculated and experimental values 

 of y. The sum of the ± actual errors is in nearly all cases zero. 



