TRANSACTIONS OF SECTION B, 573 



special consideration. Tt does not seem right to me to say even the little I can 

 say about valency witbout naming with the respect they deserve from us the dis- 

 linguished chemists who laid the foundations of the doctrine and developed it : 

 "Williamson, Odling, AVurtz, Edward Fraukland, and Kekule. I hud the good 

 fortune to be in the same laboratory as, and then intimate with, Kekule, when in 

 1854 he was working out the bivalency of sulphur and oxygen by his investigation 

 of thioacetic acid, some time, that is, before he had thought out the benzene ring 

 and the valency of carbon. 



Only when, as is usual, propositions are made in which a separate and inde- 

 pendent existence, with valency as a property, is imputed to a Judical, does the 

 question, as to what valency is, present, any difhculty. Approaching it from the 

 side of the molecule and of double decomposition, and therefore from the experi- 

 mental side instead of from that of the radical itself, as is customary, valency 

 presents itself as being the number of single interactions necessary, in order to 

 have a certain radical occur, first as that of one substance, and then as that of 

 another which has no other radical in common with the first substance. That 

 ammonia possesses one atom of the radical nitrogen, and three atoms of the 

 radical hydrogen, and that the nitrogen radical is tervalent and the hydrogen 

 radical univalent are statements mutually based upon facts such as the following. 

 I'otassium nitrilosulphate, which contains nitrogen but no hydrogen, is converted 

 by water in a sharply defined single interaction, into potassium hydrogen sul- 

 phate, and into potassium imidosulpbate, a substance which contains all the 

 nitrogen along now with hydrogen. This salt passes, also sharply and by a single 

 interaction with water, into as much more sulphate along now with potassium 

 amidosulphate, which latter substance contains all the nitrogen and twice as much 

 hydrogen as belonged to the imidosulpbate, Lastly, the amidosulphate inter- 

 acting with water gives a third quantity of potassium sulphate, equal to the last, 

 and also ammonia, having all the nitrogen of the nitrilosulphate started with, three 

 times as much hydrogen as the imidosulpbate, and nothing else. That is to say, 

 the lutrilosulphate and the ammonia have no other radical than the nitrogen the 

 same, while three single interactions have been necessary to separate in this way 

 the nitrogen radical from the three atoms of the potassiumsulphooyl radical. 

 Therefore' the nitrogen radical is tervalent and its quantity is the atom. Again, 

 there are three atoms of the univalent hydrogen radical in the ammonia molecule, 

 because in each of the three interactions an equal quantity of this radical is 

 brought in from water. Ammonia shows only one pair of radicals, behaving, so 

 far as its own interactions go, exclusively as a compound of amidogen and 

 hydrogen, and these radicals are referred to as united or Dound together in being 

 ammonia. It is only the intiiractions of its derivatives, primary, secondary, and 

 tertiary, that are indicated by treating the amidogen as ultimately nitrogen and 

 two hydrogen radicals. But this involves the consideration of all three hydrogens 

 as bound to the nitrogen ; and it becomes, therefore, of vital importance to bear in 

 mind that the hydrogen radical, proper to the ammonia itself, is bound to a 

 nitroEren radical which carries also bound to it two other hydrogen radicals. 



Chemical formulas still remain to be considered. They are symbolisations of 

 deductions from experimentally ascertained facts, and are independent of the inter- 

 pretation commonly given to them as referring to the minute differentiated 

 structure of substances. A chemical equation expresses a chemical change 

 quantitatively by means of chemical formula which are molecular. In a case of 

 double decomposition, therefore, there are four formuUc ; but when two or more 

 such interactions are expressed in one equation, because they occur together, the 

 formulae of transition-substances do not appear, and then numerals before formulae 

 tell the number of interactions in which separate molecules of the substance have 

 taken part. A formula represents the relative interacting quantity or molecule of 

 a substance, while the single symbols composing it stand each for an atom of the 

 radical of a certain simple substance as possessed by the substance formulated. 

 The connecting lines and dots, and certain collocations of the symbols, indicate 

 the association of the simple radicals as compound radicals iu different inter- 

 actions. 



