MUTUAL INFLUENCE OF THE SCIENCES. 221 



possible to calculate the rate of decreasing density in the 

 upper parts of the atmosphere ; it also became possible tc 

 make approximate tables of the atmospheiic refraction of 

 light. Thus optics, and with it astronomy, advanced with 

 barology. After the discovery of atmospheric pressure 

 had led to the invention of the air-pump by Otto Guericke; 

 and after it had become known that evaporation increases 

 in rapidity as atmospheric pressure decreases ; it became 

 possible for Leslie, by evaporation in a vacuum, to produce 

 the greatest cold known ; and so to extend our knoAvledge 

 of thermology by showing that there is no zero within 

 reach of our researches. When Fourier had determined 

 the laws of conduction of heat, and when the Earth s tem 

 perature had been found to increase below the surface 

 one degree in every forty yards, there were data for in 

 ferring the past condition of our globe ; the vast period 

 it has taken to cool down to its present state; and the 

 immense age of the solar system a purely astronomical 

 consideration. 



Chemistry having advanced sufficiently to supply the 

 needful materials, and a physiological expei iment having 

 furnished the requisite hint, there came the discovery of 

 galvanic electricity. Galvanism reacting on chemistry dis 

 closed the metallic bases of the alkalies, and inaugurated 

 the electro-chemical theory ; in the hands of Oersted and 

 Ampere it led to the laws of magnetic action ; and by its 

 aid Faraday has detected significant facts relative to the 

 constitution of light. Brewster s discoveries respecting 

 double refraction and dipolarization proved the essential 

 truth of the classification of crystalline forms according to 

 the number of axes, by showing that the molecular con 

 stitution depends upon the axes. In these and in numer 

 ous other cases, the mutual influence of the sciences has 

 been quite independent of any supposed hierarchical order. 

 Often, too, their inter-actions are more complex than as 



