PREFACE. 



century it will be found impossible to separate 

 them. 



Our questions and our experiments intersect 

 in numberless curved -lines the straight line 

 that leads to truth. It is the points of inter- 

 section that indicate to us the true direction ; 

 but, owing to the imperfection of the human 

 intellect, these curve lines must be pursued. 

 Observers in chemistry and physics have the 

 eye ever fixed on the object which they seek 

 to attain, one may succeed, for a time, in 

 following the direct line ; but all are prepared 

 for circuitous paths. Never doubting of the ul- 

 timate success of their efforts, provided they ex- 

 hibit constancy and perseverance, their eager- 

 ness and courage are only exalted by difficulties. 



Detached observations, without connexion, 

 are points scattered over the plain, which do 

 not allow us to choose a decided path. For 

 centuries chemistry presented nothing but these 

 points, although sufficient means were available 

 to fill up the intervals between them. But per- 

 manent discoveries and real progress were only 

 made when chemists ceased to make use of 

 fancy to connect them. 



My object in the present work has been to 

 direct attention to the points of intersection of 

 chemistry with physiology, and to point out 

 those parts in which the sciences become, as it 



