On an Instrument for ascertaining Refractive Indices. 509 



time the box was taken away, and the fat was now found quite 

 hard and brittle. I have not been able as yet to obtain any of 

 this substance in order to examine the changes it had under- 

 gone. In a fat of a few years old it would be always difficult 

 to show the change into stearine by an analysis of the elements, 

 for the change could not as yet be very advanced, and the 

 composition of the fat would not therefore be very different 

 from that of stearine. The conditions necessary to the above- 

 mentioned change are the same in the mine as in the cellar 

 where the box stood ; in both water was present, but not a free 

 passage of air. The change in the first two cases had been 

 going on for a long time, for it can be shown that the fat had 

 been in the mines during more than a hundred years. If this 

 one condition could be supplied by another, so that the fat 

 could be changed readily by an artificial process into stearine, 

 it would not be necessary to consider the elaine as a disagree- 

 able addition and to connect it to a bad and cheap product, 

 but the whole substance could be worked as stearine, or from 

 the whole quantity stearic acid could be obtained. 



The substances described prove the truth of the supposi- 

 tion which Liebig has made in his last treatise " On the pro- 

 duction of Fat," that liquid fats can be changed into hard 

 ones. 



LXII. On certain Improvements in the Instrument, invented 

 by the late Dr. Wollaston, for ascertaining the Refracting 

 Indices of Bodies. By John Thomas Cooper, Esq.* 



THE ordinary physical characters of substances such as 

 hardness, colour, lustre, fracture, specific gravity and some 

 others, have been the means which chemists and mineral- 

 ogists have long been in the habit of employing for the iden- 

 tification both of inorganic and organic substances; and I 

 think it will be acknowledged by all who give their attention 

 to such matters, that any additional means added to the above 

 modes of observation, if it be capable of being put into practice 

 with equal facility, and with a certainty of giving results with 

 a degree of precision little, if at all inferior to that which is 

 capable of being attained by the balance, is a sufficient apology 

 for occupying a short time of the Chemical Society. 



About forty years ago, the late Dr. Wollaston described in 

 the Philosophical Transactions, an ingenious instrument by 

 which the refractive indices of substances, either in a solid or 

 liquid state, could be with facility determined, the method re- 



* Communicated by the Chemical Society; having been read April 18, 

 1843. 



