112 Chemistry. [Lecture 28. 



I. Animal Heat is only used in the regulation 

 of the thermometer, by putting it in the mouth. 



II. Friction or Percussion is applied to some 

 .purposes in common life, as in fire-arms, where 

 fire is produced by the percussion of the flint 

 against the steel. 



III. The Flash of Electricity has not been much 

 attended to by chemists, though its effects on 

 metals are great, as you have seen in the lecture 

 on electricity. The justly celebrated Davy has, 

 however, latterly applied the kindred influence 

 Galvanism, with great success in chemical investi- 

 gation; and in no instance more so than in ascer- 

 taining its powers in effecting the decomposition 

 of different substances ; supporters of combustion 

 or acidifying principles being attracted by the 

 positive pole of the voltaic or galvanic apparatus, 

 from which oxygen is liberated, whilst inflam- 

 mable substances are attracted by the opposite 

 electrised or negative pole, from which hydrogen 

 gas is liberated. 



IV. Mixture. This heat, like the former, is too 

 transient to be applied to any useful purposes. 

 We may, however, account mixture as one of 

 the useful means of lessening heat occasionally, 

 and of producing cold. To obtain low degrees of 

 heat, we have recourse to a mixture of water or 

 pounded ice with salts ; of these muriat of lime 

 with snow is most powerful. Snow or ice mixed 

 with strong nitrous acid produces also a great 

 degree of cold, even such as to freeze the mer- 



