TO PHYSIOLOGY AND PATHOLOGY. 47 



scarcely ever absent. A change in certain vital processes of the organism, is 

 accompanied by a corresponding change in the nature, quantity, and character of 

 the combinations, which are secreted by the kidneys. It then remains for che- 

 mists to express, quantitatively, the relations in which these bodies are observed to 

 stand to each other, and to the processes in the organism. 



THE METHOD PURSUED BY CHEMISTRY TO EXPRESS THESE 



RELATIONS. 



Chemistry first tests, by analysis, the quantitative signification of the terms urea, 

 uric acid, allantoin, and oxalic acid ; by this, however no reciprocal relation is 

 established between them, and it is only by an investigation into the changes which 

 the combinations of those bodies, that have a share in the formation or change in 

 the organism, suffer under the influence of oxygen and water, that a definite and 

 undeniable connection can be numerically established. By the addition of oxygen 

 to uric acid, three products are separated, viz ; allantoin, urea, and oxalic acid. 

 By a greater addition of oxygen, uric acid passes into urea and carbonic acid. 

 Allantoin appears as an urate of urea. The comparison of the relations dis- 

 covered by chemists, in the transition of uric acid into urea, with those which 

 accompany the same process in the organism, has led to the conclusion that the 

 requirements (in this case being an addition of oxygen) are in both cases either 

 similar, or they deviate from each other. And these deviations furnish a new 

 starting point for investigations, which lead to the explanation of the process. 



Urea and uric acid are products of the changes which the nitrogenous consti- 

 tuents of the blood suffer, under the influence of water and oxygen. The nitro- 

 genous constituents of the blood are, in their composition, identical with the nitro- 

 genous constituents of nutrition. The relations of the latter to uric acid, and to 

 urea and the oxygen of the air and the elements of water are expressed in Che- 

 mistry by formulae, which explain them as far as they can be applied. 



WHAT IS MEANT BY CHEMICAL FORMULAE. 



It must be evident, even to the unlearned, that the difference in the properties of 

 two bodies, is either dependent upon a different arrangement of the elements of 

 which they consist, or upon a quantitative variety in their composition. Ghemical '"( 

 formulae are expressions of the different methods of arangement, the quantitative I 

 differences which attend the qualitative. Chemistry, even at the present day, 

 cannot by the most careful analysis establish with certainty the composition of an 

 organic body, if its quantitative relation cannot be gained from a second, which 

 has already been ascertained without any doubt ; without such aid the formulae for 

 the oil of bitter almonds and fousel oil could not have been obtained, and if the 

 relation of dependence between two bodies cannot be ascertained by direct obser- 

 vation, the chemist is obliged to find it by his analytic art ; this he does by separat- 

 ing the body into two or more products, investigating those which he obtains from 

 the action of oxygen, chlorine, alkalies, or acids ; and by the aid of these he suc- 

 ceeds finally in obtaining one or more products, the composition of which he is 

 acquainted with, and whose formulae he consequently knows. To the formulae for 

 these products, he joins that of the body which he has analysed. The sum of 

 the whole is thus obtained by aid of the knowledge of one, several, or all the parts 

 of which the aggregate consists. Thus if the number of equivalents of the carbon, 

 hydrogen, and oxygen, which appertain to a molecule of sugar, are not defin- 

 able by analysis ; and if the skill of the chemist affords no guarantee for the cor- 

 rectness of his analysis of salicine or amygdaline ; the analysis may be tested by 

 the fact, that sugar combines with oxide of lead ; and resolving itself into carbonic 

 acid and alcohol by fermentation offers two combinations, the formulae of which 

 are known ; amygdaline resolves itself into prussic acid, oil of bitter almonds, and 

 sugar. Salicine into sugar and saligenine. 



IMPORTANCE OF FORMULAE. 



It is clear that when the weight of a body, and that of one or two, or all thi 



