20 SCIENTIFIC AMUSEMENTS. 



then combines with oxygen, whence the term "double combina- 

 tion." 



Double Evolution is the name applied to a less frequently 

 occurring kind of chemical action where there are still two original 

 substances and two different products, and where the action is 

 the exact opposite of double combination; thus, suppose that 

 carbon dioxide and sulphur dioxide could so act on each other 

 that the oxygen contained in each was set free whilst the carbon 

 and sulphur combined together forming carbon disulphide, then 

 this would be an action of the kind in question. In point of fact 

 this particular change does not take place ; but certain chemical 

 actions are known of a similar nature, i.e., where two compounds 

 each containing oxygen react on one another in such a fashion that 

 oxygen is evolved from each, the other constituents of each 

 original compound either combining together, or being further 

 changed by subsidiary chemical actions. 



On the whole, the essential difference between physical and 

 chemical actions leading to change of state may be thus tabulated. 



Physical Action.* "Where one substance only is employed which becomes 

 more or less completely changed into another from which the first 

 can be reproduced at will by varying the temperature or pressure, or 

 both ; as where water becomes ice or steam, either of which will repro- 

 duce the original water by altering the temperature, raising it in the 

 first case, and lowering it in the second ; or where ammonia gas is 

 compressed to a liquid by the simple application of a considerable 

 degree of pressure, means being employed to prevent any alteration of 

 temperature during the action. 



Chemical Action. 1. Where one substance gives rise to two (or more) 

 products entirely different from each other and from the original 

 substance, by simple decomposition or breaking up. 



2. Where two substances coalesce or combine to form a single product 

 different from either, by combination or synthesis. 



8. Where two substances react on one another so as to give rise to two 

 products dissimilar amongst themselves and each different from either 

 of the original substances. This kind of action may be further dis- 

 tinguished as changes of 



a. Single displacement (Expts. 9 and 10), 



b. Double displacement (Expts. 11, 12, and 13), 



c. Double combination (Expts. 242, 243, and 244) ; 



d. Double evolution ; 



in each case the total action being capable of being regarded as the 

 result of a series of changes of decomposition and combination occur- 

 ring in succession. 



* A peculiar kind of action termed allotropic modification, and generally 

 classed as a chemical change, occasionally occurs. In this, one substance 

 becomes changed by heat or other agency into something quite different 

 from the original body, e.g., oxygen and ozone. 



