

SCIENCE, EDUCATION, RELIGION 



in which the different substances are present, by 

 the relative volatility, etc., of the bodies which 

 may be formed, and by various other conditions, 

 for *:i discussion of which reference must be made 

 to special treatises on chemistry. It is found 

 that, as a general rule, those elements most 

 readily combine together which exhibit the 

 greatest differences in their properties. Chlo- 

 rine, for example, readily combines with sodium 

 or antimony, but has very little attraction for 

 bromine, which it closely resembles in many of 

 operties. It is found, moreover, that 

 combination always takes place in certain defi- 

 nite proportions, and not in any haphazard 

 quantities. For example, 16 parts by we'ght of 

 oxygen always combine with 2 parts of hydro- 

 form 18 parts of water, and 35.5 parts 

 of chlorine always combine with 1 part of hy- 

 drogen to form 36.5 parts hydrochloric acid. 

 Indeed, it is possible to find by experiment for 

 each element a number which always represents 

 the proportion by weight in which it combines 

 with other elements. This is termed its com- 

 binint] or atomic weight. Further, it is found 

 that if a piece of iron is placed in a solution of 

 sulphate of copper, metallic copper is deposited 

 on the iron, whilst a portion of the latter is dis- 

 solved, and for every 63.4 parts of copper depos- 

 ited, 56 parts of iron are always dissolved. Again, 

 when iron is placed in dilute sulphuric acid, 

 hydrogen gas is given off and the metal is dis- 

 solv. '!. and it is found that for every 1 part of 

 hydrogen given off, 28 parts of iron are dissolved. 

 It follows that 56 parts of iron are capable of 

 replacing, or are chemically equivalent to, 63.4 

 parts of copper or two parts of nydrogen. Many 

 examples of a similar kind might be quoted. 

 Briefly, it is found that a certain definite quan- 

 tity of each element is capable of combining 

 with, or of replacing in compounds, certain 

 <! -finite quantities of other elements, and these 

 are termed their chemical equivalents. For the 

 sake of comparison, one part of hydrogen is 

 usually taken as the standard to which all other 

 equivalents are referred, and from the example 

 ibove it is evident that the equivalent of 

 28, and so on. In some cases, the num- 

 ber representing the equivalent of an element is 

 the same as that representing its atomic weight; 

 many cases, for reasons which cannot be 

 1 into here, the latter is some simple 

 multiple of the former. The c^nirnlrnt is a 

 quantity determined by actual experiment, the 

 weight is to a certain extent a matt- T of 

 theory. A similar series of facts is obset 



6 <>f rompMiind-. I'or example, 56.1 parts 



s-.li will neutrali/e as much acid as Id part- 



of soda or 17 part- nf ammonia; 50.1 parts of 



potash, 40 parts of soda, and 17 parts of am- 



nre therefore chemically equivalent to 



ther. Chemistry was formerly divided 



brandies; lnr. ; *tr\f, OF the 



try of the mineral kingdom, and Organic 



or the chemistry oi the vegetable and 



annual kingdom ' liOSC substance . which 



are produced by vital action. It was I- 



that then- wa- -ice between 



1 that it was impossible to prepare 



artificially in the laboratory those compound- 



i in the bodies of plants and animal-. 



In 1828, however, the substance urea, a body 

 essentially characteristic of vital action, was 

 prepared artificially, and even built up from its 

 elements, by the German chemi.-t. \Yohler. Since 

 that time a large number of the compounds 

 found in plants and animals have been produced 

 from inorganic substances, or built up from 

 other organic bodies, and it is now known that 

 there is no essential difference whatever between 

 organic and inorganic chemistry. The same 

 forces are at work in both, subject to the same 

 laws. One element, however, is contained in 

 all organic bodies, viz, carbon. Carbon has the 

 peculiar property of combining as it were with 

 itself, and by virtue of this property it gives 

 j rise to an enormous number of deriva ! 

 some of very complex composition and consti- 

 tution. Simply for the purposes of study, these 

 are still classed together under the head of 

 Organic chemistry, which is defined as the cfu-m- 

 istry of the carbon compounds. The greater num- 

 ber of the carbon compounds now known are 

 artificial products which do not occur in nature. 

 The majority of them may be regarded as de- 

 rived from the hydro-carbons by the replace- 

 ment of one or more atoms of hydrogen by some 

 other element or group of elements. Amongst 

 the most important of the series thus derived are 

 the haloid derivatives, alcohols, ethers, acids, alde- 

 hydes, ketones, and amines. There are, ho 

 important groups of substances, the relation- 

 ships of which are not yet clearly made out. 

 Amongst these are the carbo-hydrates and the 

 nlhtlnids. The constitution and relationships 

 of the proteids or album inoil substances, and of 

 some others found only in the bodies of plants 

 or animals, are still less understood. Notwith- 

 standing the differences already alluded to, 

 chemical attraction is closely related to the 

 various forms of energy, and, indeed, is itself 

 a form of potential energy. Of late years, the 

 study of the changes in the distribution of energy 

 which accompany chemical change has 1> 

 of great importance. Chemical combination is 

 in the majority of cases accompanied by devel- 

 opment of heat, and the quantity of heat thus 

 developed by the formation of a given 

 of a particular substance is always the same, 

 and the decomposition of any compound re- 

 quires the e\iH-nditure. in the form of heat or 

 otherwise, of exactly the same amount of energy 

 as was liberated by its formation. ( >n the other 

 hand, the formation of some compounds is at- 

 tended by absorption of heat, and exactly the 

 same amount of neat is liU-rated when the com- 

 pound decomposes. That branch of the 

 which deals specially with the development or 

 obsorption of heat which accompanies <!,. 

 reaction* is termed tlnrmorchcm\tr\j. It is found 

 that those compounds in the formation of which 

 the irreate-t amount ot hr.it i- de\ .-1. .( H-d Of 



free are the most stl the most difficult 



to decompose, and vice versa. Further, when 



several sub-lanres which tJWorrtfcoib r:.]. I-a.l 



in several dim-rent ways, producing several 



different compounds, an-" mixed together 



alwavH found that those bodies are produced in 



the formation "t \\hich the greatest amount of 



pad This important law is known 



lie principle of maximum work. It has also 



