CHEMICAL AFFINITY.] 



CHEMISTRY. 



301 



As we proceed, many other applications of heat will be 

 noticed, which will be more properly dealt with as they 

 occur. 



The agency of light, in producing chemical changes, is 

 more restricted than that of heat, and has been already 

 enlarged on under the head of Photography. The recent 

 discoveries of Messrs. Bunsen and Kirchoff, which have 

 been also described, have opened a new connection 

 between the relation of light and chemical phenomena. 



Electricity has great influence on the attraction or 

 affinity which one body has for another. It will be 

 unnecessary for us to enter into that question, however, 

 as it has been fully discussed under the head of Voltaic 

 Electricity and Electro-Metallurgy ; and the same remarks 

 apply equally to magnetism. Such being the relation 

 which the undulatory forces have to chemical attraction, 

 we may now more fully inquire into its different phases 

 and law, and so prepare our readers to consider the 

 numerous instances to which we shall have to draw their 

 attention as we proceed. It may be explained, that certain 

 terms are used to express the changes produced by che- 

 mical action. By decomposition, we mean the separation 

 of existing compounds ; and by composition, the union of 

 bodies to form new compounds is expressed. When the 

 chemist attempts to ascertain the constituent parts of 

 body, the process is termed analysis ; but the act of 

 producing new combinations, is termed synthesis. 



It has been stated that different substances have 

 various powers of affinity for each other ; and we shall 

 now suggest some experiments illustrating this fact. 

 We jn ust, however, point out, that there are two classes 

 of affinity; or, more properly, a double act of affinity 

 takes place under some circumstances. Thus, if a piece 

 of polished iron be exposed to the action of air, its 

 oxygen unites with the metal to form oxide, or rust ; 

 and here we have only an action of simple affinity. If, 

 however, we add together solutions of common salt and 

 nitrate of silver, then a double affinity operates, for the 

 silver unites with the chlorine of the common salt ; 

 whilst the nitric acid combines with the soda ; and so 

 two new substances the chloride of silver, and the 

 nitrate of soda are produced. The following are in- 

 stances in which the chemical affinity existing between 

 two or more substances, attended with the removal or 

 separation of another by decomposition, is involved. 



Ex/Kriment 1. Add a few drops of a solution of 

 nitrate of silver to one of common salt. The silver, 

 uniting with the chlorine, as already mentioned, will 

 fall down as a white powder ; and a solution of nitrate 

 of soda will form the liquid. 



Experiment 2. Pour off the clear solution into a 

 clean glass vessel, and add to it a few drops of sul- 

 phuric acid. The nitric acid will be driven away from 

 the soda, with which the sulphuric acid combines to 

 form sulphate of soda. 



Experiment 3. Pour the solution of sulphate of soda, 

 obtained in the last experiment, into a glass vessel, and 

 add to it a little nitrate of baryta in solution. The sul- 

 phuric acid will now leave the soda, and combine with 

 the baryta, forming sulphate of baryta ; whilst the soda 

 will combine witli the nitric acid set free from the nitrate 

 of baryta, and so form nitrate of soda. 



In these three experiments, we observe one substance, 

 the soda, changing its state thrice. First, we found it 

 in union with chlorine, forming common salt, which is 

 left, owing to the superior affinity for it which nitric 

 acid has. In the second experiment, we find that it left 

 the nitric acid for the sulphuric acid, owing to the more 

 powerful attraction which the latter sxarcised. But, in 

 the third experiment, we noticed the paradoxical fact, 

 that it united with nitric acid in preference to sulphuric 

 acid. But this is easily explained. The baryta in the 

 nitrate of that earth has a greater affinity for sulphuric 

 acid than any other body, and so it decomposed the sul- 

 phate of soda seizing the acid. Now, the soda being set 

 free in the presence of the nitric acid, at once combined 

 with it ; and in this we have an instance of double decom- 

 position, and a twofold exertiou of chemical attraction. 



Our readers will do wisely to master the principles 



involved in these experiments before proceeding further ; 

 and we have chosen the substances named that they 

 may trace the progress of the soda in each of its com- 

 binations. To make the matter still clearer, we add 

 diagrams representing the results, ic., of each experi- 

 ment, iu which the materials employed, and the new 

 compounds formed, are mentioned. 



Materials used. 



Common J 

 Salt. 1 



Nitrate of / 

 SUrer. 1 



Experiment 1. 



Chlorine 



Nitric Acid 

 Silver 



Experiment 2. 



New Compounds formed. 

 Nifnte of Soda. 



Chloride of Silver. 



Nitrate of 



Soda. 



Nitric Acid 

 Ml 



-Nitric Acid. 



Sulphuric Acid 



=J Sulphite of Soda. 



Experiment 3. 



Sulphate of /Sulphuric Acid . 

 ^ I Soda- 



Nitrate of f 

 Baryta. | 



Nitric Acid 

 Baryta 



Nitrate of Soda. 



Sulphate of BaryU. 



We thus perceive how much the affinities which one 

 body has for another vary, and how the affinities of a 

 third may destroy those exhibited in previous compounds 

 when it is presented iu proper circumstances. 



In the above instances, we have confined our attention 

 to changes easily understood. As we advance, however, 

 into those which are produced when vitality assists the 

 process, the conditions are so varied, and the states of 

 bodies involved are so numerous, as to render the matter 

 far more complex. Take, for instance, the ordinary 

 process of digestion, and the conversion of bread and 

 meat into blood and muscular fibre. Here we have a 

 delicate and astonishing adjustment of affinities. The 

 food contains charcoal, oxygen, hydrogen, and nitrogen, 

 besides mineral substances. Part of the charcoal goes 

 separately to form blood, fat, muscular fibre, nails, hair, 

 skin, (tc. With some of the oxygen it affords carbonic 

 acid, which passes off from the lungs. The hydrogen, 

 nitrogen, and oxygen are also employed to form various 

 parts of the animal substance ; and all these bodies, when 

 in excess, are driven off in mutual combination as water, 

 ammonia, or carbonic acid. Yet, complex though these 

 changes may be, still the same law governs them as is 

 found active in our experiments. Each body has a 

 definite attraction for another ; and the knowledge of 

 this enables the chemist to trace, by the eye of science, 

 the most obscure and difficult details. 



There are some minor circumstances which powerfully 

 assist in promoting ths exercise of chemical affinity, to 

 which we shall now allude ; and they refer chiefly to 

 peculiar conditions in which bodies may be placed with 

 respect to their external surface, or relative mecha- 

 nical conditions. As an instance of this, we have the 

 peculiar action which the metal platina exercises on a 

 mixture of oxygen and hydrogen gases ; the action of 

 light on a mixture of chlorine and hydrogen ; the effect 

 of minutely divided substances, as spongy platina, pow- 

 dered charcoal, metal in a minute state of division ; as 

 in the case of finely divided luad in pyrophorus, (to., 

 (tc. Such, and many other instances we might name, 

 affect, vary, and modify many of the exercises of che- 

 mical affinity ; and all such must be taken into account 

 in chemical researches, if truth and accuracy of result 

 are hoped for. It will be thus perceived, that a thorough 

 chemist must exercise the greatest care and penetration 

 in all cases that are brought before his notice ; and the 

 science, therefore, is not only valuable for its philosophical 

 and commercial results, but equally so as a system or 

 means of mental training. 



In cases of extremely low temperature, chemical at- 

 traction is often entirely suspended. Thus, on Mont 

 St. Bernard, the bodies of deceased travellers b>Ye been 



