CHEMISTRY. 



tion, but acids and bases enter into combination 

 with water in another and special way. We have 

 pointed out before that there are substances which 

 may be said to be intermediate between acids and 

 bases, which act as bases to strong acids, and as 

 acids to strong bases. Now, water is such a sub- 

 stance. Water unites with many bases, forming 

 substances called ' hydrates,' or ' hydrated bases ' 

 (thus, ' hydrate of lime,' ' hydrated oxide of lead,' 

 &c.) ; it also unites with many acids, forming sub- 

 stances called ' hydric salts,' or ' hydrated acids ' 

 (thus, 'hydric sulphate/ or 'hydrated sulphuric 

 acid,' &c.). This kind of combination of water 

 with acids and bases was long confounded with 

 one or other of the two kinds mentioned just before, 

 and from this there has arisen a confusion in the 

 nomenclature of acids, bases, hydrates, and hydric 

 salts, which is very likely to prove perplexing to 

 the beginner. 



Thus, the term ' sulphuric acid ' is used to 

 express two different things I. Oil of vitriol, 

 which, when as strong as it can be made, con- 

 sists of nearly pure ' hydric sulphate,' or ' sulphate 

 of water ; ' and 2. A white, solid substance, which 

 is oil of vitriol without the water. This white 

 solid, when put into water, unites with it, -giving 

 out a great deal of heat, and forming oil of vitriol, 

 or ' hydric sulphate.' 



In the case of many other acids, we have the 

 same double meaning of the word ; we have the 

 * anhydrous ' (that is, waterless) acid, and the 

 compound of that with water the hydrated acid, 

 or hydric salt. Some chemists restrict the word 

 acid to the hydric salt, and call the anhydrous 

 acid the ' anhydride ; ' others restrict the name 

 acid to the anhydrous substance, and call the 

 hydrated acid a hydric salt. Thus, we have the 

 name sulphuric acid applied to two substances 

 anhydrous sulphuric acid, or sulphuric anhydride ; 

 and hydrated sulphuric acid, or hydric sulphate. 

 And these differ from one another in that 

 the latter is a compound of the former with 

 water. 



As all the processes described in the preceding 

 pages in illustration of the action of acids on 

 other substances, were assumed to take place in 

 water, it was not necessary to allude to this 

 ambiguity, and the acids taken as examples 

 (sulphuric, nitric, hydrochloric, and acetic) were 

 supposed to be in the state of hydric salts. The 

 question, indeed, would be one of only theoretical 

 interest, if every acid existed in both forms. But 

 there are some acids which occur only as hydric 

 salts, others which occur only as anhydrous acids. 

 Thus, carbonic acid gas dissolves in water, but 

 does not combine with it to form a hydrated acid, 

 or hydric carbonate. Again, oxalic acid cannot 

 be obtained in the anhydrous form. If we try to 

 drive off the water from hydrated oxalic acid 

 (hydric oxalate), we find that the acid itself de- 

 composes and breaks up into other substances. 

 We shall return to the consideration of these 

 relations when we come to speak of salt- 

 radicals. 



As acids (that is, anhydrous acids) unite with 

 water to form hydrated acids, or hydric salts, 

 compounds in which water acts as a base, so 

 anhydrous bases unite with water also, forming 

 hydrated bases or hydrates in which water acts 

 as an acid ; and here also we are apt to get into 

 confusion with the names. Thus, the word lime 



i3 generally used to mean quicklime the an- 

 hydrous base, but not unfrequently for what is 

 properly called slaked lime, the compound of lime 

 and water (hydrate of lime). 



Having considered in a general way the char- 

 acters of acids and bases, we shall now shortly 

 examine their composition, for they are all com- 

 pounds that is, they can all be produced by the 

 union of elements, and can all be decomposed or 

 broken up by separating these elements from each 

 other. And first, we shall investigate the com- 

 position of water, which is, as we have seen, both 

 an acid and a base an acid in relation to the 

 strong bases; a base in relation to the strong 

 acids. 



Water, as is well known, was long regarded as 

 a simple or elementary substance. It is one of 

 the four 'elements' of the ancients fire, air, 

 earth, and water ; and although we now know 

 that water is decomposed in a great many chemi- 

 cal changes constantly taking place, and produced 

 from its elements in some of the most familiar 

 and frequent cases of chemical action, the real 

 nature of these changes escaped the observation 

 of chemists until a comparatively recent time. 

 The discovery of the composition of water was 

 made by Cavendish about the year 1781. He 

 shewed that when hydrogen gas is burned in the 

 air, water is formed ; and that if, instead of com- 

 mon air (which contains about 20 per cent, of 

 oxygen), pure oxygen is used, the hydrogen and 

 oxygen disappear, and water is produced, and 

 that the weight of the water formed is the same as 

 that of the hydrogen and oxygen which have dis- 

 appeared. He thus proved that water is formed 

 by the union of hydrogen and oxygen that it is 

 a compound of these two gases. By measuring 

 the quantity of each gas,* he ascertained the pro- 

 portion in which they unite, and shewed that one 

 volume of oxygen unites with two volumes of 

 hydrogen. As oxygen weighs bulk for bulk 16 

 times as much as hydrogen, the proportion by 

 weight is 8 parts of oxygen to I of hydrogen ; 

 that is, I oz. of hydrogen in burning will use up 

 and unite with 8 oz. of oxygen, and produce 9 oz. 

 of water ; or 9 oz. of water can be decomposed 

 into 8 oz. of oxygen and I oz. of hydrogen ; and 

 the i oz. of hydrogen will occupy a space twice 

 as great as the 8 oz. of oxygen. The composi- 

 tion of water was discovered by the method of 

 synthesis that is, by forming it from its con- 

 stituents. We can also prove it by analysis 

 that is, by decomposing it into its constituents. 

 This may be done by passing through water a 

 current of electricity, from a galvanic battery for 

 instance, when the hydrogen will bubble up from 

 the one wire (that connected with the zinc of the 

 battery), and the oxygen from the other (that 

 connected with the copper of the battery). The 

 two gases can thus be collected separately, and 

 measured, when it will be found that the hydrogen 

 produced fills just twice the space filled by the 

 oxygen. 



Water is thus a compound of hydrogen and 

 oxygen, or, in chemical language, an oxide of 

 hydrogen ; the compounds of oxygen with other 

 elements being termed 'oxides.' Now, a very 



When the bulks of two or more gases have to be compared, 

 they must be measured under the same pressure and at the same 

 temperature, as a change of pressure or of temperature produces a 

 change in the bulk of a gas. 



