In regard to the influence of chemical combination two general laws may be laid down. One is, that _poisons which only act locally, have their action much impaired or even neutralized, in their chemical combinations_. Sulphuric acid and muriatic acid on the one hand, and the two fixed alkalis on the other, possess a violent local action; but if they are united so as to form sulphates or muriates, although still very soluble, they become merely gentle laxatives. But the case is altered if either of the combining poisons also act by entering the blood. For the second general law is, that _the action of poisons which operate by entering the blood, although it may be somewhat lessened, cannot be destroyed or altered in their chemical combinations_. Morphia acts like opium if dissolved in alcohol or fixed oil; if an acid be substituted as the solvent, a salt is formed which is endowed with the same properties: The sulphate, muriate, nitrate, acetate of morphia all act like opium. Strychnia, arsenic, hydrocyanic acid, oxalic acid, and many more come under the same denomination: Each produces its peculiar effects, with whatever substance it is combined, provided it do not become insoluble. Mr. Blake has recently laid down what may be considered a branch or corollary of the second of these general propositions, and has confirmed it by many appropriate experimental facts,—namely, that _the salts of the same base produce the same actions, independently of the acids with which they are combined_.[50] The law, however, is a more general one, as given above, and was stated in former editions of the present work. It applies not only to bases, but likewise to acids, such as the hydrocyanic, oxalic, arsenious, and arsenic acids, and also to neutral organic principles which act through the blood, such as picrotoxin, colocynthin, elaterin, and narcotin. The same author considers it to be also a probable conclusion from a variety of experiments on the salts of various bases, that _those salts which are isomorphous, or possess the same crystalline form, are closely allied in action_.[51]