318 A Sketch of the Laws of Chemical Combination. 



which liave been already alluded to; {ox, whilst in some instances it acts as 

 a base to acids, it appears to hold the relation of an acid to most of the 

 salifiable bases, and forms with them quaternary compounds, which may 

 fairly be considered as salts. This view is confirmed by the fact, that if a 

 hydrate be decomposed by the voltaic battery, the base is found at the 

 negative pole, whilst the water passes to the positive. Its affinity for the 

 pure alkalies is so strong, that no degree of heat will drive it off; and, 

 though most of the earths may be obtained free from it, a very considerable 

 elevation of temperature takes place during their combination with it, as 

 is seen every day in the slaking of lime. For the other metallic oxides, its 

 affinity is very variable, but in most cases less forcible. It is the opinion 

 of Berzelius, that the quantity of oxygen in the water of a hydrate is the 

 same as that which exists in the oxide with which it combines ; so that a 

 protoxide will combine with one atom of water, a deutoxide with two, and 

 so on. If this opinion be correct, it shows another strong analogy between 

 the hydrates and other compounds acknowledged as salts. Many other 

 combinations of the metals with sulphur, chlorine, iodine, cyanogen, &c. 

 have a greater or less affinity for water. A familiar example of this is 

 afforded by the blue and green sympathetic inks, which are composed of 

 the chlorides of cobalt and nickel : both of these compounds are of a pale 

 colour when in the state of hydrate ; but if the water be driven off by a 

 moderate degree of heat, their colour is rendered much brighter, but fades 

 again by the absorption of water as they cool. 



The combinations of water with acids, as a salifiable base, have been 

 investigated as yet in a few cases only ; but the results already obtained, 

 principally by Mr. Graham, are very interesting, and open a new path for 

 investigation. It has been shown that phosphoric acid cannot be obtained 

 except in combination with three equivalents of water, which is termed 

 basic, and which may be wholly or partly replaced by other salifiable 

 bases. On exposing this compound to heat, one atom of water is driven 

 off, and an acid remains, having the same proportions of phosphorus and 

 oxygen as exist in phosphoric acid, but combined with only two equiva- 

 lents of water, and altered in several of its chemical relations ; this, which 

 is termed paraphosphoric acid, being exposed to a still higher temperature, 

 is converted by the loss of another atom of water into metaphosphoric 

 acid, which differs in 'ts properties from the two former, but retains the 

 same atomic constitution. These three acids are therefore isomeric ; the 

 first throws down a yellow precipitate with oxide of silver, and has a ten- 

 dency to unite with three atoms of water, or an alkaline base ; the second 

 gives a snow white granular precipitate, and combines with two atoms of 

 a base; the third produces a gelatinous precipitate, and unites with only 

 one atom of a base. Arsenic and telluric acids have a tendency to form 

 combinations similar to those of phosphoric acid ; and the ordinary con- 



