SPECIFIC GRAVITY. 81 



It will be seen from Table I. that the specific gravity of 

 ordinary fats varies from 902 '83 to 904 '56, while the spe- 

 cific gravity of butter-fat as exhibited in Table II. rarely 

 falls below 910, the usual range being from about 911 to 913. 

 There is thus a material difference between the specific 

 gravity of butter-fat and that of ordinary commercial animal 

 fats, and it is obvious that this difference affords a satisfac- 

 tory basis upon which to found an additional test for deter- 

 mining the purity of butter. 



Dr. Dupre has shown that the specific gravity of fats may 

 be much affected by previous treatment ; nevertheless, we con- 

 sider it a very valuable test, and in cases where large num- 

 bers of butters are passing through the analyst's hands, may 

 be used, together with the microscope examination, as a 

 means of selecting those samples which shall be more fully 

 examined. 



We cannot help regretting that no mention is made in the 

 tables of the microscopic appearances of these butters. Such a 

 column would have been of use, we think, in accounting for 

 the remarkably high percentage of water found in many of 

 the samples ; as we have shown in the table given at page 

 19, we find that when water is present to an amount of 

 more than 10 or 12 per cent., the sample usually appears 

 to have been fused. This, and the fact that we have 

 never yet met with a butter, known to be pure, with any- 

 thing like so much water as many of Mr. Bell's samples 

 evidently had, compels us to the belief that he placed too 

 much faith in the agents which he employed to collect his 

 samples. 



Furthermore, we think the choice of the point 100 F. 

 was a most unfortunate one. Even forgetting the fact, that 

 most chemists have adopted the Centigrade scale, and that 

 most scientific books give only degrees C., we think that 

 100 F.=:37'8 C. is far too low to be convenient. There 



