XVJii PREFACE. 



to truth. It is the points of intersection that indi- 

 cate 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 space, 

 in following the direct line ; but all are prepared for 

 circuitous paths. Never doubting of the ultimate 

 success of their efforts, provided they exhibit con- 

 stancy and perseverance, their eagerness and cou- 

 rage are only exalted by difficulties. 



Detached observations, without connection, are 

 points scattered over the plain, which do not allow 

 us to choose a decided path. For centuries chemis- 

 try presented nothing but these points, and sufficient 

 means were available to fill up the intervals be- 

 tween them. But permanent 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 at- 

 tention to the points of intersection of chemistry with 

 physiology, and to point out those parts in which 

 the sciences become, as it were, mixed up together. 

 It contains a collection of problems, such as chemistry 

 at present requires to be resolved ; and a number 

 of conclusions drawn according to the rules of that 

 science from such observations as have been made. 



