468 FORMULAE, CHEMICAL 



acid, one atom of hydrogen replaced by the oxidised radical acotyle, C 4 H 8 7 , and 

 the other by one atom of basic hydrogen. By ' basic ' hydrogen is meant, that it acts 

 tho part of, and can be replaced by, a metal. The opinions of chemists with regard 

 to the nature of the radical existing in acetic acid are divided. Some consider the 

 acid as tho hydrated teroxide of tho non-oxidised radical acetylo (C 4 K S ), and therefore 

 write its formula C 4 H 3 8 + HO. But as tho chloride of the oxidised radical can be 

 isolated, we cannot doubt its existence. Moreover, there is no doubt of the existence 

 of the other radical, C'H 3 , because wo find it replacing hydrogen in tho base acety- 

 lamine. See ACETYLAMINE. But tho conclusion must be drawn from these facts 

 that there aro two radicals, one existing in acetic acid C 4 H 3 2 , which Williamson 

 calls othyle, and another, sometimes called vinyle C 4 H 3 , which exists in aldehyde, in 

 olefiant gas, and several other bodies. The radical in acetic acid is, consequently, not 

 C 4 H 3 , but C 4 H 3 2 . 



The next illustration is that of alcohol, which consists of two atoms of water, in 

 which one atom of hydrogen is replaced by ethyle, and the other by hydrogen. Ether, 

 on the other hand, is derived from the same type, both atoms of basic hydrogen being 

 replaced by ethyle. Hydrate of potash and anhydrous potash will, after what has been 

 said, explain themselves. It will be seen that in all these illustrations, the same 

 vapour-volume is preserved, and by this means the apparent anomaly of ether and 

 alcohol being of different vapour-volumes is removed. While the type two atoms of 

 wator ( = 4 volumes) has an actual existence, it remains for chemists to discover 

 whether wo are justified in receiving as types bodies which have no real existence, 

 such as throe atoms of water. 



Type, two atoms of hydrogen. The type ammonia has already been sufficiently 

 illustrated ; it remains, then, only to show what substances are to be regarded as 

 formed on the type hydrogen. M. Gerhardt, in addition to these, adopts hydrochloric 

 acid as a type ; but when we consider that that acid is itself formed on the hydrogen 

 model, it appears unnecessary to raise it to the dignity of a separate type. 



Two atoms of Olefiant Marsh Hydrochloric Prussic Chloride of 



hydrogen. gas. gas. acid. Benzole. acid. ethyle. 



iP C 4 H 3 C 2 H 3 Cl 



H H H H 



The above will be sufficiently plain after what has been said, it being remembered 

 that C-H 3 is methyle, C 4 H 5 ethyle, C 12 H 5 phenyle, and C 2 N cyanogen. 



It is a fruitful source of annoyance to students and others to find, on looking through 

 chemical works, the same substance represented by different authors with totally dif- 

 ferent formulae. We .shall endeavour to give a few instances and such explanations as 

 will assist in enabling tho student to overcome the difficulty. It is often the case that 

 the differences in the formulae arise from the works consulted having been written at 

 different dates ; the older one is then, in most cases, to be rejected, because it is pro- 

 bable that the formulae in it have been corrected by subsequent and more accurate 

 researches. It not unfrequently happens that an author writes nitrous acid NO 4 , and 

 the true nitrous acid (NO 3 ) is called hyponitrous acid. It may serve to assist the 

 student in correcting any errors on this point, to consult a list of the oxides of 

 nitrogen according to the nomenclature at present employed: such a list will be found 

 in the article NITROGEN. 



A still more common cause of difficulty is to be found in the different theoretical 

 views of certain chemists regarding the constitution of certain chemical substances. 

 It is explained under tho article ATOMIC WEIGHTS that many of tho elements have 

 within tho last few years had their atomic weights doubled ; and this alteration 

 has of course greatly affected chemical formulae. But even with the same atomic 

 weights, the formula of a given substance may admit of considerable variety of ex- 

 pression, which becomes a source of perplexity to those who are not accustomed to 

 such changes. It may be instructive to show in how many ways the formula of a 

 simple compound like sulphuric acid may bo written, according to the different views 

 which may bo taken of its constitution. Thus, it may bo supposed that this acid is a 

 compound of water (IPO) 1 and sulphuric anhydride (SO 3 ) ; hence the formula of sul- 

 phuric acid may be written IPO.SO 3 . Another chemist may regard this acid as ft 

 compound of a radical (SO 4 ) with hydrogen ; for example, when sulphuric acid is acted 

 upon by zinc, the zinc replaces the hydrogen, which is set free, and zinc sulphate is 

 formed ; hence we may formulate sulphuric acid as IPSO 4 . But by the reaction of 

 peroxide of barium and sulphuric acid, peroxide of hydrogen (H 2 a ) is set free ; hence 

 there is a reason for writing its formula, IPO'.SO*. Yet another mode of expressing 



1 It will be convenient for the purposes of this illustration to employ the modem atomic weights, 

 = 10, 8 = 32, 



