CHEMICAL CONSTITUTION AND PHYSIOLOGICAL ACTION. 153 



the following conditions : — 1st, That it is unambiguous ; that is, that the change 

 of structure produced by it is susceptible of only one interpretation. 2d, That 

 the change of structure produced by the operation is, in all cases investigated, the 

 same, and the change of constitution (AC) — that is, the change of structure and 

 potential— as nearly as possible the same. 3d, That the operation is completely 

 under our control, so that it cannot be either performed or reversed spontaneously, 

 in ordinary circumstances, within the animal body. 4:th, That the substance is 

 equally suitable for absorption into the system before and after the change (that 

 is, that <£ and <£ + A$ are observed under similar conditions) ; and 5th, That a 

 decided change of physiological action is, in some cases at least, produced (that 

 is, that A<J> is not always = 0). 



Chemical operations may be divided into two classes — 1st, operations of sub- 

 stitution ; and 2d, operations of addition or subtraction. In the first, an atom or 

 group of atoms is replaced by an equivalent atom or group of atoms, without any 

 change taking place in the active atomicity of any atom or radical in the 

 substance. 



In the case of addition (and by subtraction we mean to express merely the 

 inverse operation to addition), the active atomicity of one or more atoms or 

 radicals in the compound is increased, and the bonds thus set free, or rendered 

 active, are saturated by atoms or radicals (the sum of whose active atomicity is 

 of course an even number), which are thus added to the substance. We shall 

 apply the name condensation to capability of being added to in whatever way the 

 addition takes place, and distinguish two kinds of condensation, intra-atomic and 

 inter-atomic ; in the first of which it is an atom, and in the second a compound 

 radical, the active atomicity of which is increased. Thus, carbonic oxide, sulphide 

 of methyl, and protochloride of tin, are examples of intra-atomic condensation; 

 olefiant gas, the dibasic anhydrous acids, and allylic alcohol, of inter-atomic con- 

 densation ; while hydrocyanic acid (if we assume for it the formula 0-0Z0 ) 

 shows both. 



Many operations of addition and also of substitution satisfy the 1st, 2d, 

 3d, and 4th of the five conditions mentioned above; but when we examine 

 them in reference to the 5th condition, we find a marked difference. Operations 

 Of substitution (satisfying the 1st, 2d, 3d, and 4th conditions) do not appear 

 greatly to change the physiological activity of a substance, except, 1st, where the 

 activity depends on direct local action ; or 2d, where the operation removes or 

 introduces an atom or radical, the compounds of which are as a rule active. As 

 examples of the first exception, we may take sulphuric acid (H 2 S0 4 ) and caustic- 

 soda (HNaO), both poisonous ; while sulphate of soda (Na 2 S0 4 ) and water (H 2 0) 

 are not : as examples of the second, acetate of lead and cyanide of sodium, both 

 poisonous, acetate of potash and chloride of sodium not. Besides the exceptions 

 which can be reduced to the two classes just mentioned, there are several isolated 



