340 



Mr. J. Dewar on the Atomic Volume 



free state from the volume of the oxide, gave, approximately, the 

 same value of 5'2 in the oxides of copper, zinc, cadmium, lead, 

 mercury, iron, cobalt, and titanium. In other words, the 

 oxygen occupied the same volume in each combination. Other 

 classes of oxides gave a volume of twice or half the above 

 number. In order to arrive at the volume of the oxygen, 

 Schroter started with the premises that the metal in the com- 

 bined state occupied the same volume as the uncombined metal. 

 Granting, for the present, that oxygen has a definite volume in 

 combination in the oxides, it is clear that the volume obtained 

 by difference will vary with the volume of the combined metal. 

 The same method applied to the oxides of the less dense metals 

 would give a negative volume to the oxygen ; and in these cases 

 we must admit condensation to have taken place in the metal 

 itself. We may have three cases, therefore, according as the vo- 

 lume of the combined metal differs or not from that of the un- 

 combined. If it remains the same in combination, we obtain the 

 real volume ; if it condenses, the volume is a minimum ; if it 

 expands, a maximum. Seeing that the oxygen in the dense 

 metals has the volume 5*2, we may regard the greater or 

 smaller volume obtained from some oxides as the result of con- 

 densation or expansion of the metal. Supposing the above 

 volume (5*2) to exist generally in the oxides, we should have a 

 condensation in the less dense metals in combination, approach- 

 ing very nearly, in the case of potassium, sodium, and alumi- 

 nium, to one-third, and in calcium, magnesium, and strontium 

 to nearly one-half, of the volume in the free state. Thus far, 

 then, this number would give a rough explanation in admitting 

 condensation in many of the metals. 



I have thought that it would be interesting to compare this 

 volume with the volume of oxygen when it is combined with 

 solid substances other than metallic, and to take a series of 

 analogous combinations. For this purpose the chlorine family 

 are well fitted in their respective combinations with potassium, and 

 these with oxygen. The following Table contains the best 

 known density determinations and volumes of chloride, bro- 

 mide, and iodide of potassium, compared with the densities of 

 chlorate, bromate, and iodate. 





Specific 

 gravity. 



Volume. 



Difference. 



Mean vol. of 

 0(=I6). 



KC1 



1-977 



2-326 



2-69 



3-271 



30 



3-979 



37-61 

 52-6 J 

 44 1 

 51 1 

 55-31 

 537 [ 



15 



7 



-1-6 



5 

 23 



-0-5 



KC10 3 



KBr 



KBrO 3 



KT 



KIO 3 





