10 CHEMISTRY AND PHYSICS IN RELATION 



muscle, may be brought about by different.causes, and that one and the same cause 

 may bring about a variety of effects. 



CHEMICAL COMBINATION. 



We know that the simple process of chemical combination is dependant upon at 

 least three causes or conditions, which must stand in certain relation to each other, 

 if the combination is to be formed, and that affinity, the force of cohesion, and 

 heat, have an equal share in the process. 



DIFFERENT EFFECTS OF HEAT. 



We know further that when a given quanity of heat expands a solid body, and 

 forces its minutest parts to separate from each other, a double or triple quantity 

 will entirely change the properties of the body, and that a further alteration occurs 

 in these properties if the amount of heat that is communicated exceed a certain 

 degree. 



It is perfectly certain that expansion, liquefaction, and transition into the gaseous 

 form are dependant upon causes, identical in their nature, but that the effects 

 produced are by no means proportional to the causes ; the reason of this has been 

 justly sought in the reaction or resistance of some other cause, and our idea of 

 the existence of the power of cohesion thus acquires a more scientific basis. 



The same degree of heat, which is a condition of the combination of the oxygen 

 of the air with mercury, produces the opposite effect the decomposition of the 

 oxide of mercury into mercury and oxygen, if the temperature be raised a few 

 degrees. 



By a simple process of oxidation we derive acetic acid from alcohol : we obtain 

 this acid from the oxidation of salicylite of potash; we may also exhibit it from 

 wood, sugar, and starch, by the mere application of heat and the exclusion of the 

 oxygen of the atmosphere : in all these cases the product yielded is the same ; but 

 the conditions of its formation are extremely different. 



THE SEPARATION OF VITAL EFFECTS, AND THE CHIEF REQUISITES 



THERETO. 



If it be true that physiology can only attain to a scientific basis by the investi- 

 gation of the plurality of conditions, on which the phenomena of life depend ; and 

 if it be granted that this can only be attained by a consideration and separation of 

 vital effects, and the conditions to which they give rise ; it is evident that since a 

 number of causes have, or may have, an influence upon these effects, the physiol- 

 ogist ought to possess an intimate knowledge of all the forces and causes which 

 may bring about changes of form and character in matter ; since, without this, he. 

 would be unable to separate true effects from those which might be erroneously 

 ascribed to the cause, and which, perhaps, have nothing in common with indications 

 of gravity, affinity, &c. 



CONTINUED DISREGARD OF THESE PRINCIPLES. 



No one can deny that these principles are applied in the investigations of 

 pathology at the present day, and the difference between the method of inquiry 

 now pursued from that in use in the earlier stages of philosophical science is 

 cerUinly \ery great, although the influence of the older system is not quite exier- 



