FORMULA, CHEMICAL 467 



suppose it bo necessary to express the fact that one equivalent of oxide of lead added 

 to one equivalent of sulphuric acid produces sulphate of lead, we write, PbO + SO 8 

 forms sulphate of lead. But it is more usual and brief to put down the terms con- 

 nected by the plus sign followed by the sign of equality, and then the formula of the 

 resulting compound, thus: PbO + SO 3 = PbO.SO 8 . A collection of symbols ex- 

 pressing the nature of a reaction or decomposition, the two terms being united by the 

 symbol of equality, is called an equation. Equations are of the highest value to the 

 chemist, as enabling him to express in the simplest possible manner the most com- 

 plicated reactions. Moreover, these equations enable us to see at a glance the true 

 nature of a decomposition. To take a simple case, namely, that of the decomposition 

 of terchloride of antimony by carbonate of ammonia, we have 



SbCl 3 + 3(NH<0,C0 2 ) = SbO + 3NH<C1 + 3C0 2 . 



Or, in words, terchloride of antimony plus three equivalents of carbonate of ammonia, 

 yields one equivalent of teroxide of antimony, three of chloride of ammonium, and 

 three of carbonic acid (carbonic anhydride). 



The above illustrations will suffice to show the principles upon which formulae and 

 equations expressive of chemical decompositions are constructed. In writing equa- 

 tions showing the metamorphoses of substances with which it may be supposed the 

 reader of them may not be very fully acquainted, it is proper to place beneath them 

 the names of the substances in full ; thus : in writing the change supposed to be ex- 

 perienced by amygdaline under the influence of a ferment which does not itself con- 

 tribute any substance to the reaction, we might say : 



4HO = C H 6 2 + C 2 NH + 2C 12 H 12 



Amygdaline Bitter almond Prussic Grape sugar, 



oil. oil. acid. 



In writing the formulae of substitution-compounds, it is convenient to place the 

 replaced and replacing substances in a vertical line, so as to indicate at a glance the 

 substitution which has taken place. As an illustration, we shall place side by side 

 the chemical type ammonia and some bodies derived from it by substitution. 



{H fC 2 H 3 fC 2 H 3 fC 2 H 3 fC 12 H 3 fpt fC 2 H 3 



H N^ H N^ C 2 H 3 N<^ C 2 H 3 N^ H ]& pt P<^ C 2 H 3 

 H L H L H l.c 2 H 3 L H LH lc 2 H 3 



Ammonia. Methylamine. Bimethy- Trimethy- Aniline. Platina- Triphosphme- 

 lamine. lamine. mine. thylamine. 



In the first of the above formulae we have the type or starting point, ammonia itself. 

 In the next we find one atom of hydrogen (two volumes) replaced by ono atom (tv,-o 

 volumes) of the radical methyle. In the third we find two atoms of hydrogen 

 replaced ; and in the fourth illustration all three have been replaced by methyle. The 

 fifth formula is that of ammonia, in which one atom of hydrogen is replaced by 

 phenyle, forming phenylamine, or, as it is more usually termed, aniline. The sixth 

 illustrates a very peculiar substitution. In it we find two atoms of hydrogen replaced 

 by the platinicum of the late illustrious chemist, M. Gerhardt, who regarded platinum 

 as entering into substitutions with two atomic weights, as if it were two metals. The 

 one being the platinum of chemists generally, its atomic weight being 99 (and its 

 symbol Pt) ; this he calls platmosum. The other being platinicum (pt), with an atomic 

 weight half that of platinosum, namely, 49'5. The last formula is that of the singular 

 base, triphosphmethylamine. In it we see the nitrogen of the original type replaced 

 by phosphorus, and each atom of hydrogen by methyle. 



It will^ greatly assist the student in his endeavours to recollect chemical formulae, if 

 he commits to memory the principal types, and the substances which are regarded as 

 formed on their model. The following are those which are best established : 



Type, two atoms of water. This typo is written in such a manner that the replace- 

 ment of the hydrogen can be distinctly seen. By its side are placed a few of the 

 substances formed on the same model. 



Two atoms Hydrate of Anhydrous 



of water. Acetic acid. Alcohol. Ether. potash. potash. 



In the above simple illustrations of the type water we have, in the case of acetic 



HH 2 



