ATTRACTION 165 



expressed by 1, that of oxygen will be 8, that of carbon 6, that of 

 sulphur 16. If hydrogen and oxygen unite in one proportion of each, 

 the compound will be expressed by 9, this is the number repre- 

 senting water, and every combination of water will be expressed by 

 9, or 18, or 27, or 36, and so on, even to ten proportions, which 

 would be expressed by 90. 



(ww.) The combining weight or power of a body being once ascer- 

 tained, it will always remain the same ; or it will sustain the same ra- 

 tio in every combination. If the combination takes place in different 

 proportions with a given body, the number expressing the lowest pro- 

 portion will be constant, and the higher proportions will be multiples 

 of it, by a whole number. Thus, hydrogen being unity, oxygen will 

 always enter into combination in the proportion 8, 16, 24, 32, &c. ; 

 carbon in the proportions 6, 12, 18, 24, &ic. The combining weights 

 or powers of bodies, both simple and compound, may therefore be per- 

 manently registered in a table of chemical equivalents. Such a ta- 

 ble is now attached to every treatise on chemistry, and is constantly 

 referred to in practical operations, both of science and art. It is an 

 important auxiliary, for we discover by inspection what quantities 

 of particular bodies saturate, or are equivalent to each other. In 

 the present work the chemical equivalents, as far as they are ascer- 

 tained, will be found connected with each body, in its proper place, 

 and they will be collected in a table at the end.* 



Dr. 'Wollaston 1 s Scale of chemical equivalents.^ This is a table of 

 combining or proportional weights, embracing those bodies that are 

 most frequently used in practical chemistry. It differs from other 

 tables only in this, that while the names of the substances are station- 

 ary, those of the numbers are placed on a sliding rule, divided logo- 

 metrically, according to the principle of that of Gunter. The advan- 

 tage of the instrument is, then, that it not only presents a table of 

 chemical equivalents, but by moving the sliding rule in a proper man- 

 ner, many proportions can be mechanically worked, without the 

 trouble of calculation. Thus, it has been already stated, that sul- 

 phate of potassa is composed of acid 40+ potassa 48, and therefore 

 88 is the number expressing the composition of the salt ; hydrogen 

 being the unit, all this will be seen, by placing the scale in such a po- 

 sition that 8 is opposite to oxygen ; but if we wish to know what 

 would be the proportion of the acid and alkali, in 100 parts of sul- 

 phate of potassa, we have only to bring the scale into such a posi- 

 tion, that 100 will be opposite to sulphate of potassa, when we shall 



* A very valuable table is annexed to Dr. Thomson's First Principles of Chem- 

 istry, and Mr. Brande has published, in a separate work, the equivalents of all bodies 

 as far as they are known. 



t For a description of this beautiful instrument, see the Phil. Tr. for 1814. 



