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NATURE 



[Oct. 17, 1889 



igreater affiaity to ths chalk gas? Will llmj displace sodium 

 -oticle from soda or vice v;rsd ? O.i adding lima witer to soda 

 solution, a precipitate of chalk is formed. W.iat does the 

 ■solution contain? Lime water contains lima in conbinxtioi 

 with water ; is the sodium oxide present in combination with 

 water? Soda is bailed with milk of lime (in an iron sauce- 

 pan to avoid breakage) until it no longer affec'.s lime water ; 

 afterwards the liquid is poured off and boiled doA^n. The 

 product is very unlike soda : it is very caustic, and when 

 •exposed to the air becomes liquid. If it is an analogous sub- 

 stance to slaked lime, it should combine with chilk gas and be 

 reconverted into soda ; this is f )und to be the case. Caustic soda 

 is thus discovered. Chalk and lime are known to neutralize 

 acids ; bo"h soda and caustic soda are found to do so, and their 

 effect on vegetable colours is found to be the reverse of that of 

 acids. At this stage the origin of the name alkali is explained, 

 and it is pointed out that the oxides which have been studied 

 may be arranged in two groups of alkali-like or alkylic and acid- 

 forming or acidic oxides, the former being derived from metals, 

 the latter from non-metals. The production of salts by the 

 union of an oxide of the one class with the oxide of the other 

 ■class is then illustrated by reference to earlier experiments. 



The point is now reached at which the results thus far 

 ■obtained may be reconsidered. The student has been led in 

 many cases to make discoveries precisely in the manner in which 

 they were originally made ; and it is desirable that at this 

 •stage, if not earlier, the history of the discovery of the compo- 

 sition of air and water, &c., should be briefly recited. It is 

 then pointed out that a variety of substances have been 

 • analyzed and resolved into simpler substances — air into oxygen 

 and nitrogen, water into oxygen and hydrogen, &c. ; and 

 that these simpler substances thus far have resisted all 

 ■attempts to farther simplify them, and are hence regarded as 

 ■elements. A list of the known elements having been given, the 

 diverse properties of the elements may be illustrated from the 

 knowledge gained in the course of the experiments. The fact 

 that when eleman'.s combine compounds altogether different in 

 properties from the constituents are form ad also meets with 

 manifold illustratio.a. Too little has been ascertained to admit 

 ■of any general CDUclusion being arrived at with regard to the 

 proportions in which elements combine, but it is clear that they 

 may combine in more than one proportion since two oxides of 

 •carbon have been discovered, and in the only cases studied — 

 viz. copper oxide and chalk — the composition has been found 

 not to vary. The existence of various types of compounds has 

 been recognized, and a good deal has been learnt with reference 

 to the nature of chemical change. Bat, above all, the method 

 ■of arriving at a knowledge of facts has been illustrated time after 

 time in such a manner as to influence in a m>st important degree 

 'the habit of mind of the careful sttident. New facts have been 

 ■discovered by the logical application of previously discovered 

 facts : the logical use of facts, and the habit of using facts have 

 been inculcated. This is all-important. It has become so 

 eustomary to teach the facts without teaching how they have 

 been discovered that the great majority of chemical students 

 never truly learn the use of facts ; they consequently pursue their 

 daily avocations in a perfunctory manner, and only in excep- 

 tional cases manifest thDse qualities which are required of the 

 investigator; their enthusiasm is not awakened, and they have 

 little desire or inclination to add to the stock of facts. It must 

 not for one moment be supposed that the object of teaching 

 ■chemistry in schools is to make all chemists. Habits of 

 ■regulated inquisitiveness, such as must gradually be acquired by 

 all who intelligently follow a course such as has been sketched 

 •out, are, howev^er, of value in every walk of life ; and certainly 

 the desire to understand all that cooies under observation shjuld 

 as far as possible be implanted in everyone. 



Stage V. — The quintitative stage. 



The quantitative composition of many of the substances which 

 (have previously been studied qualitatively should now be 

 •determined — in some cases by the teacher in face of the pupils, 

 •so that every detail may be observed and all the results re- 

 corded ; in other cases by the pupils. 



The composition of water is first determined by Dumas' 

 method; this may easily be do le, and fairly accurate results 

 may be obtained in the coarse of a couple of hours. The 

 ^results obtained by Dumas and subsequent workers should then 

 all be cited, and, attention having been drawn to the extent to 

 •which such experiments are necessarily subject to error, the 



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evidence which the results afford that hydrogen and oxygen 

 combine in certain y?jr^(/ and invariable proportions to form water 

 is especially insisted upon. 



The composition of chalk gas is next determined ; this also is 

 easily done, as impure carbon (lampblack) may be burnt and 

 the hydrogen allowed for. Again, attention is directed to the 

 results obtained by skilled workers, and the evidence which they 

 afford that chalk gas never varies in composition. 



The composition of copper oxide has already been ascer- 

 tained ; it may be re-determined by reducing the oxide in 

 hydrogen : in fact, in determining the composition of water. 



The lead oxides may be reduced in a similar manner, the 

 oxide obtained by igniting white lead as well as red lead and 

 the brown oxide obtained by treating red lead with nitric acid 

 being used. In this way it is ascertained that the brown oxide 

 is the highest oxide ; the loss in weight which this oxide suffers 

 when ignited may then be determined. 



Tabulating the results thus obtained, after calculating with 

 what amount of the particular element that quantity of oxygen 

 is associated which in water is combined with one part by 

 weight {unit 7veigkt) of hydrogen, numbers such as the following 

 are obtained : — 



I part of hydrogen is combined with 8 parts of oxygen in 

 water. 



3 parts of carbon are combined with 8 parts of oxygen in chalk 

 gas. 



3f5 parts of copper are combined with 8 parts of oxygen in 

 copper oxide. 



103 '5 parts of lead is combined with 8 parts of oxygen in lead 

 oxide (litharge). 



5 1 '8 parts of lead are combined with 8 parts of oxygen in lead 

 oxide (brown). 



These clearly illustrate the fact that elements combine in very 

 different proportions, and the results obtained with the lead 

 oxides aiford also an illustration of combination in. multiple 

 proportion. 



The amounts of silver and lead nitrates formed on dissolving 

 silver and lead in nitric acid are next determined by evaporating 

 the solutions of known weights of the metals in porcelain 

 crucibles on the water-bath, and then drying until the weight is 

 constant ; accurate results may be easily obtained, and these 

 two exercises afford most valuable training. The nitrates are 

 subsequently evaporated with muriatic acid and the weights of 

 the products determined. What are these products ? Does the 

 metal simply take the place of the hydrogen in hydrogen chloride 

 as zinc does when it dissolves in muriatic acid ? If so, the products 

 are silver and lead chlorides, and it may be expected that the 

 same substances will be obtained — that the same increase in 

 weight will be observed, when, say, silver is combined directly 

 with chlorine as when it is dissolved in nitric acid and the 

 solution is precipitated with muriatic acid or salt. Silver is, 

 therefore, heated in chlorine, and is found to increase in weight 

 to the same extent as when it is dissolved in nitric acid, &c. ; a 

 given weight of silver precipitated by salt is also found to 

 increase to the same extent as when it is directly combined with 

 chlorine. The composition of silver chloride having thus been 

 ascertained, the amount of chlorine in salt is determined. 

 The composition of salt being ascertained, purified dried 

 washing-soda is converted into salt, and also the amount 

 of chalk gas which it contains is determined : from the data, the 

 composition of sodium oxide mayJbe calculated. In like manner 

 the composition of lime may be ascertained by converting chalk 

 into chloride by igniting it in hydrogen chloride, and then 

 determining the chlorine in the chloride ; the same method ma 

 be applied to the determination of the composition of the oxid 

 and chlorides of zinc, magnesium, and copper. 



Discussing these various results, and comparing the quantitie's 

 of oxygen and of chlorine which combine with any one of the 

 metals examined, it is seen that in every case about 35^4 pari 

 of chlorine take the place of 8 parts of oxygen. Com bins 

 tion in reciprocal proportions is thus illustrated, and by considei 

 ing the composition of chalk and washing-soda it may be show: 

 that this applies equally to compounds of two and to compounds 

 of three elements. As 35 4 parts of chlorine are found in every 

 case to correspond to 8 parts of oxygen, it is to be expected 

 that hydrogen chloride contains one part of hydrogen in 

 combination with 35*4 parts of chlorine ; a solution containing 

 a known weight of hydrogen chloride is, therefore, prepared b. 

 passing the gas into a tared flask containing water and t' 

 chlorine is then determined. 



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