CHEMISTRY 



153 



will IM- precipitated, and only sodium 

 nitrate will remain in solution. The action may 

 be represented by an equation : 



NaCl + AgNOj = NaNO, + AgCl (precipitate). 



The action of sulphuretted hydrogen on many 

 metallic solutions illustrates double decompositions 

 in which the action is complete, as, 



HgCl a + H,S = HgS + 2HC1, 



where the mercuric sulphide formed is insoluble in 

 water, and is consequently obtained as a pre- 

 cipitate. 



In connection with the subject of double decom- 

 position the bearing of the law of Kichter (already 

 mentioned in the historical sketch ) may be illus- 

 trated. Looking at the quantitative signification 

 of the following equations, 



KC1 + AgN0 3 = AgCl + KNO., 

 NaCl + AgNO, = AgCl + NaNO 3 , 



we sfto that the quantity of chlorine which was 

 united with 39 parts by weight of potassium or 

 23 of sodium to form a salt is exactly the quantity 

 required to form a salt with 108 parts by weight of 

 silver, whilst, similarly, the quantity or the group 

 N0 8 which was united to these 108 parts by weight 

 of silver is exactly the quantity required to form a 

 salt with 39 parts by weight of potassium or 23 of 

 sodium. The same holds good generally for double 

 decompositions. 



Another very important kind of chemical change 

 is the displacement of one element in a compound 

 by another. Chlorine, for instance, displaces the 

 iodine in potassium iodide and takes its place : 



2KI + C1 2 = 2KC1 + I 2 . 



The greater affinity of potassium for chlorine than 

 for iodine is the explanation given of this displace- 

 ment. Displacement of one metal by another is a 

 familiar phenomenon, although the chemistry of 

 what is taking place may not be familiar to all who 

 have seen it. When a piece of bright iron or steel, 

 as a key or the blade of a knife, is dipped into an 

 acidulated solution of cupric sulphate ( olue vitriol), 

 a reddish deposit of metallic copper is formed 

 almost immediately upon the surface of the metal. 

 This copper is derived from the cupric sulphate 

 solution ; but what is not manifest from observa- 

 tion alone, is that at the same time an equivalent 

 quantity of iron is dissolved away and goes into 

 solution as ferrous sulphate. The action is, 



CuSO 4 + Fe = FeSO< + Cu. 



The whole of the copper would eventually be 

 separated from the solution in the metallic state if 

 enough iron were present, and for every 63 parts of 

 copper precipitated 56 parts of iron would go into 

 solution. 



Inorganic and Organic Chemistry. The whole 

 subject of chemistry has been divided into two 

 great divisions, named respectively inorganic 

 and organic. Made originally to separate from 

 each other the chemistry of purely mineral sub- 

 stances, and that of substances of animal or 

 vegetable origin, which were at the time sup- 

 posed to be capable of formation only as products 

 of vital processes, this subdivision is retained still 

 mainly as a matter of convenience. The division 

 of organic chemistry is sometimes spoken of now as 

 the chemistry of the compounds of carbon ; but 

 this is not a very strict definition, as many carbon 

 compounds occur in nature as purely mineral sub- 

 stances, and having really no connection with 

 organic chemistry, such as numerous mineral car- 

 bonates. As has been already stated, it is mainly 

 for convenience that the consideration of the 

 majority of the compounds of carbon is taken as a 

 separate branch, not because of any difference in 



the chemical principles involved, but really on 

 account of the very great number of thene com- 

 pounds, and of the great complexity of many of 

 them. 



It is in the domain of organic chemistry that the 

 study of the constitution of nultttances has been 

 most diligently prosecuted, and with the greatest 

 amount of apparent success. The graphic formula 

 which chemists assign to acetic aciu (to take a 

 simple example) is, 

 H 



H C C H. This formula is adopted in order 



to express a number of ideas concerning the sup- 

 poseu mode of arrangement of the atoms in acetic 

 acid, deduced from the study of its formation, its 

 decompositions, and the action upon it of various 

 substances. The known facts find suitable expres- 

 sion in the formula, and there is no observation 

 yet made as to the chemical relations of acetic acid 

 which is at variance with the constitution indicated 

 by it. It would not be possible here to quote 

 evidence in favour of a particular constitution for 

 any substance, but it may be stated generally that 

 chemists endeavour to fix the constitution of the 

 simplest compounds on the firmest possible basis, 

 and, in passing from the simple to the more com- 

 plex, to make secure every step. 



The tetravalent character of the carbon atom, 

 and the great facility with which carbon atoms 

 enter into combination with other carbon atoms and 

 with the atoms of other elements, give their impress 

 to the whole of organic chemistry. The graphic 

 formulae of organic substances amply illustrate the 

 former, whilst the -syntheses of a long array of 

 simple and complex organic compounds as amply 

 illustrate the latter. 



A certain amount of knowledge of chemistry is 

 eminently useful in almost every walk of life. An 

 intelligent knowledge of the chemistry involved in 

 the processes of the kitchen, the dairy, the dye- 

 house, the farm, or the manufactory, places the 

 possessor engaged in any of these processes on a 

 different level from the rule-of-thumb worker, who 

 is as ignorant of the reason for adopting a particular 

 method as he is of the properties of the materials 

 he employs. Technical chemistry deals especially 

 with tne application of the principles and processes 

 of chemistry to the arts and manufactures, and it 

 is to those who are engaged in manufactures of 

 almost every kind that a knowledge of chemistry 

 is a particular advantage. It is not a question of 

 expediency alone, but one of absolute necessity 

 that a technical education, including chemistry as 

 one of its principal subjects, should form not the 

 least important part of the equipment for his work 

 of any artisan wno is to excel in liis employment in 

 intelligence and skill. 



In connection with this article should be read the 

 articles ATOMIC THEORY, which is to a certain ex- 

 tent supplementary to this, ANIMAL CHEMISTRY, 

 and VEGETABLE PHYSIOLOGY. See also separate 

 articles on the several elements, those on the vari- 

 ous acids, those on the great chemists, and the 

 following as amongst the most important of the 

 many chemical articles throughout this work : 



