116 CONTRACTION AND EXPANSION 



the size of the particles increases, until it ivholly 

 predominates over the repulsive force. The atoms 

 of hydrogen, nitrogen, oxygen, and light carbu- 

 retted hydrogen, retain a large amount of caloric 

 around them in proportion to their size ; the con- 

 sequence of which is, that the thermo-repulsive 

 force greatly predominates. But when they are 

 made to combine with each other chemically, or 

 with carbon, phosphorus, sulphur, chlorine, &c. 

 making gases of greater specific gravity and 

 atomic weight, their elastic force is diminished, 

 even in cases where little or no caloric is given 

 out, as in the combinations of nitrogen and hy- 

 drogen, nitrogen and oxygen, nitrogen and car- 

 bon, with numerous others, formed of these and 

 of sulphur, phosphorus, &c. 



It has long been a matter of controversy whe- 

 ther the oxygen and nitrogen of the atmosphere 

 exist in a state of mechanical mixture, as main- 

 tained by Dr. Dalton, or in a state of chemical 

 combination. Dr. Thomson has recently main- 

 tained that it is composed of 80 parts nitrogen 

 to 20 of oxygen, constituting definite proportions 

 by volume ; therefore, that it is a chemical com- 

 pound. (Records of Science, vol. iii. page 184.) 

 This hypothesis does not accord with the best 

 established facts connected with the theory of 

 volumes. For example, in all the combinations 

 of oxygen and nitrogen that are known to be che- 

 mical, there is a reduction of volume and elasti- 



