Oct. 17, 1889] 



NATURE 



603 



■characteristics of compounds are elucidated. Then the com- 

 parison may be made with air, and the fact made clear that it 

 behaves as a mere mixture. Still no reference should be made 

 to elements. 



Problem V. To determine ivhat happens ivhen organic 

 substances are burnt. — The experiments thus far made have 

 shown that phosphorus and a number of metals burn in the air 

 because they combine with the oxygen, forming oxides, heat 

 being given out as a consequence ; but that chalk when burnt is 

 split up or decomposed into lime and chalk gas, this result being 

 a consequence of the heating alone, the air having nothing to do 

 with it. It remains to ascertain what happens when organic 

 substances are burnt, as these give no visible product beyond 

 a little ashes. As in all cases when vegetable or animal sub- 

 stances are burnt a certain amount of "char" is obtained, which 

 then gradually bums away, charcoal or coke is first studied. It 

 having been discovered that the oxygen in air is the active cause 

 of burning in many cases, it appears probable that the air is 

 ■concerned in the burning of charcoal, coal, &c. As when once 

 set fire to, these continue to burn, the charcoal is at once heated 

 in oxygen : it burns, but no visible product is formed ; it therefore 

 follows that if the charcoal is oxidized the oxide must be an 

 invisible gas. How is this to be tested for ? What gases are 

 already known to the pupil ? How are these distinguished ? 

 < )xygen is excluded. Is it perhaps nitrogen, and is not perhaps 

 the nitrogen in air merely used-up oxygen as it were, pro- 

 duced by the burning of organic substances ? Or is it perhaps that 

 gas which was found in the air along with oxygen and nitrogen, 

 and which turned lime water turbid ? This last being an easy 

 test to apply is at once tried ; the lime water is rendered turbid, 

 and so to leave no doubt a sufficient amount of the gas is pre- 

 pared and passed into lime water, the precipitate is collected, 

 and the loss it suffers on heating is determined and found to agree 

 with that suffered by the precipitate prepared from chalk gas. 

 Finally, to ascertain whether the product is really heavier than 

 the charcoal burnt, as in the case of the metals previously 

 studied, the charcoal is burnt in oxygen in a tube connected 10 

 ;a flask containing milk of lime with a lime-drying tube attached 

 to it ; the tube is weighed before and after burning. Thus the 

 discovery is made that chalk gas is an oxide of carbon, and that 

 chalk consists of at least three things. 



It may be objected that to make the experiment in this manner 

 takes too much time ; but to this it may be answered that such 

 experiments are precisely of the kind of those made in actual 

 practice, and that they exercise a most important influence in 

 teaching the pupils to take nothing for granted, never to jump at 

 conclusions, and to rest satisfied if they progress surely, however 

 slow the advance may be. 



A number of organic substances may now be burnt, and the 

 gas passed into lime water ; chalk gas is found in every case to 

 be a product, and hence the presence of a common constituent — 

 carbon — in all is established. In makin;^ these experiments the 

 formation of a liquid product is observed, so it is evident that 

 chalk gas is not the only product, or carbon their only constituent. 

 Food materials generally having been found to contain 

 ""carbon," as they are obviously in some way destroyed within 

 the body, and it is known that air is necessary for life, the 

 question arises, what becomes of food, and why is air necessary 

 for life ? Is the food, perhaps, in large part " burnt up " within 

 the body, thus accounting for the fact that our bodies are always 

 warm ? The characteristic product of combustion of carbon- 

 aceous substances is therefore tested for by breathing into lime 

 water. The discovery thus made affords an opportunity for a 

 digression and for explaining how plants derive their carbon 

 from the air. 



' Problem VI. To determine what happens when sulphur is 

 burnt. — From the results of the experiments with carbon, it 

 appears pr-jbabje that the disappearance of sulphur when burnt 

 is also really due to its conversion into a gaseous oxide, so it is 

 kindled and introduced into oxygen : if it be burnt over water in 

 a bell jar in a spoon passing through the stopper (a rubber cork), 

 the water is seen to rise ; if, on the other hand, it be burnt in a 

 dry flask closed by a rubber cork carrying a gauge-tube, as 

 suggested by Hofmann,^ the volume is seen to be almost un- 

 changed after combustion. It follows, therefore, that the sulphur 

 and oxygen unite and form a soluble product. Sulphur is next 



' By burning carbon also in this way a most effective demonstration is 

 given of the fact that no loss or gain of matter attends the ch.inge, and that 

 •only heat escapes ; the results in the case of carbon and sulphur are part.cu- 

 arly striking, as the products are gaseous and invisible. 



burnt in a tube in a current of oxygen, and the gas is passed into 

 water ; a solution is thus obtained having the odour of the gas 

 and sour (acid) to the taste. The fact that carbon and sulphur 

 — both non metals — behave alike in yielding gaseous oxides 

 suggests that a comparison be made of their oxides ; so the acid 

 solution is added to lime water ; a precipitate is formed, which 

 redissolves on adding more of the sulphur gas solution ; on the 

 other hand, on adding the lime water to the acid liquid, this 

 latter after a time loses its characteristic smell. There can be no 

 doubt, therefore, that the sulphur gas doe-! in some way act upon 

 the lime. The experiment is then made of burning the sulphur 

 in a weighed tube containing lime ; the weight increases, so that 

 no doubt remains that sulphur, like carbon, forms an oxide when 

 burnt. The discovery that the addition of more of the sulphur 

 oxide leads to the dissolution of the precipitate which it first 

 forms in lime water, suggests trying the effect of excess of the 

 carbon oxide on the lime water precipitate ; this is done, and the 

 discovery is made that the precipitate gradually dissolve!. The 

 solubility of the ne^v substance may then be determined by 

 passing the gas into water containing chalk in suspension, 

 filterin<?, and evaporating. This leads to the observation that a 

 precipitate is formed on heating the liquid, and this is soon found 

 to be chalk. An opportunity is thus afforded of explaining the 

 presence of so much "chalk" in water; of demonstrating its 

 removal by boiling and by lime water ; and the effect it has on 

 soap. 



The observation that the oxides of both carbon and sulphur 

 combine with lime suggests trying whether the one will turn out 

 the other ; so the solution of the sulphur oxide is poured on to 

 chalk : effervescence is observed, and on passing the gas into lime 

 water a precipitate is obtained. The production of this effect by 

 the ac/V/ solution suggests trying common vinegar — a well-known 

 acid substance. This also is found to liberate chalk gas, and the 

 discovery of an easy method of preparing chalk gas is thus 

 made. The oxide formed on burning phosphorus, having pre- 

 viously been found to give an acid solution, is tried, and it is 

 found that it also liberates chalk gas. As a good deal of vinegar 

 is found to give very little chalk gas, the question arises, Are 

 there not acids to be bought which will have the same effect and 

 are stronger and cheaper ? On inquiry it is found that sulphuric 

 acid or oil of vitriol, muriatic acid or spirits of salts, and nitric 

 acid or aquafortis may be bought, and that these all act on chalk. 

 The behaviour of chalk with acids affords a means of testing the 

 lime water precipitate obtained in working out Problems IV. 

 and V. In this manner the pupil is led to realize that certain 

 agents may very readily produce effects which are only with difii- 

 cuky produced by heating— that the chetnical agent may produce 

 very powerful effects. The ready expulsion of the carbon oxide 

 of chalk suggests that other substances not yet studied, such as 

 the motals, when treated with acids may behave in a special 

 manner which will afford information as to their nature. At 

 this point, prior to making the experiments with the acids, an 

 explanation may be given of the names oil of vitriol, spirits of 

 salts, and aquafortis ; the processes by which they are made 

 may be described and illustrated, without, however, any attempt 

 bemg made to explain them from the chemical point of view. 

 The sulphuric acid should be made from green vitriol, and its 

 behaviour on dilution should be demonstrated as well as its use 

 as a drying agent. 



Problem VII. To determine lohit happens tvhen metals are 

 heated ivith acids. — Iron, zinc, lead, tin, copper, and silver may 

 be taken. On pouring diluted oil of vitriol on to iron or zinc, 

 the metal dissolves with effervescence ; the gas is collected, and 

 when tested is found to burn. Thus a new gas is discovered, 

 differing from all which have previously been studied, inasmuch 

 as it is combustible ; in order not to interrupt the study of the 

 action of acids on metals, however, its further examination is 

 postponed for a while. Resuming the experiments with metals, 

 lead, tin, copper, and silver are found not to be acted upon by 

 diluted oil of vitriol. 



Muriatic acid, in like manner, dissolves iron and zinc and also 

 tin with effervescence, and the gas which is given off in each case 

 exhibits the same behaviour as that obtained from iron or zinc 

 and diluted oil of vitriol. -Lead, copper, and silver are not 

 appreciably affected. 



Aquafortis is found to dissolve not only iron and zinc but also 

 copper, lead, and silver, and to convert tin into a white substance 

 — to attack all the metals in fact, thus justifying its name. The 

 gas which is given off as the metal dissolves is observed to be 

 coloured ; when it is collected over water, however, it is seen to 



