abstracts: physics 113 



Upon testing this •instrument under actual field conditions it was 

 found that the needle, on account of its rather unusual length, required 

 too frequent readjustment of balance. As a remedy for this inconven- 

 ience the point of suspension (agate cup) has now been raised, likewise the 

 ends of the needle have been bent upward to bring them again in line 

 with the suspension point. ' R. L. F. 



PHYSICS. — On the density of solid substances, with especial reference 

 to permanent changes produced by high pressures. John Johnston 

 and L. H. Adams. Journal of American Chemical Society, 34: 

 563-584. 1912. 



With a new and improved form of pyknometer the density of salts 

 and other substances has been determined with an accuracy of 3 or 4 

 units in the fourth decimal place, that is, within 0.02 per cent. In 

 many cases, however, such accuracy is unnecessary, since the variations 

 of density due to inhomogeneities of the material may be much greater 

 than this. 



Powdering a crystalline substance does not change its density by an 

 amount which we can detect with certainty, provided that the material 

 is homogeneous and free from cracks and holes ; but if the substance is 

 not homogeneous, then, as might be expected, the fine powder is denser 

 than the coarse particles. 



Neither does very high hydrostatic pressure produce any after-effect 

 on the density of strictly homogeneous crystalline compounds. 



But if the pressure be not uniform, then the density of a metal which 

 has been subjected to such compression — or has been deformed in any 

 other way — ^usually increases first (owing presumably to the filling up 

 of pores and cracks) and then decreases, sometimes even so as to reach 

 a final density less than the original value. Subsequent annealing of 

 the specimen causes a renewed increase of density. The direction of 

 the change of density on compressing bismuth is, contrary to Spring's 

 conclusion, the same as that for other metals, namely a decrease of 

 density, following upon deformation. The bearing of these results upon 

 the question of the "flow" of metals is discussed: they are shown to 

 be in harmonj^ with the idea that the "flow" — ^or indeed any deforma- 

 tion — of a metal is a manifestation of a real melting produced by the 

 unequal strains set up during the process. 



Finally, it is important to emphasize the fact the density of most 

 substances is somewhat variable, owing to a lack of complete homogen- 

 eity of the material. In consequence of this, slight changes of density 



