6c 2 



NATURE 



\Oct. 17, 1889 



quantitative similarity of the two results suggests that iron and 

 phosphorus behave alike towards air, and vice vcrsA, and serves 

 to confirm the idea that some constituent of the air present only 

 to the extent of about one-fifth is active. But nothing is to be 

 taken for granted, so iron is exposed in the phosphorus-air 

 residue and phosphorus in the iron-air residue : as no change 

 occurs, there is no room left for doubt. Recalling the experi- 

 ments in which various metals were burnt in air, in order to 

 determine whether in these cases the same constituent of the air 

 was concerned in the change, air from which the active con- 

 stituent has been removed by means of phosphorus is passed 

 through a heated tube containing bits of the metals : no change 

 is observed, so it is evident that as a rule, if not always, one 

 and the same constituent of air is concerned. The experiments 

 with iron and phosphorus, although they show that the air is 

 concerned in the changes which are observed to take place, do 

 not afford any information whether or no the water which is also 

 present is concerned in the change. Phosphorus is therefore 

 burnt in a " Florence " flask closed with a rubber stopper : on 

 removing the stopper under water some water enters, and by 

 measuring this and the amount of water which will fill the flask 

 the same result is obtained as in the previous cases. To be 

 certain whether in this case anything enters or escapes from the 

 flask it is weighed before and after the phosphorus is burnt. 

 There is no change in weight. But does nothing escape ? Yes, 

 much heat ; whence it follows that heat is not material— that, 

 although some of the air disappears, it is merely because it has 

 become affixed to or absorbed by something else. This has 

 been proved in the case of the rusting iron and the burnt metals. 

 To obtain indisputable evidence in the case of the phosphorus 

 this is burnt in a current of air in a tube loosely filled with 

 asbestos to retain the smoke : the weight is found to increase. 

 The observation that the phosphorus ceases to burn after a time 

 suggests the introduction of a burning taper into the residue left 

 by iron, &c. ; it is found to be extinguished. Then a candle 

 rnd subsequently a gas flame may be burnt in a bell jar full of 

 air over water. Reversed combustion may then be demonstrated 

 in order to fully illustrate the reciprocal character of the pheno- 

 mena. Thus it is ascertained that all ordinary cases of com- 

 bustion are changes in which the air, and not the air as a whole 

 but a particular constituent, is concerned, and no doubt remains 

 that the same constituent is always active, but active under 

 different conditions ; it is realized also that the production of 

 heat is the consequence of the union of the substance burnt with 

 the active substance in air. The experiment of exposing phos- 

 phorus in air affords the opportunity of demonstrating the 

 evolution of heat even in a case where no visible combustion 

 occurs, as the phosphorus is always observed to melt. At this 

 stage careful note should be taken of the appearance of the 

 different products of combustion, and of a change such as that 

 which occurs when the product from phosphorus is exposed to 

 the air. 



Problem III. To separate the active from the inactive con- 

 stittient of air. — It now has become of importance to get this 

 active constituent of the air by itself, and the question arises 

 whether it cannot be separated from one of the metals or other 

 substances with which it has been found to combine. The pupil 

 is therefore told to collect information about the different 

 substances formed by burning metals, &c., — whether they can be 

 obtained in sufficient quantity to work with, &c. Iron rust and 

 iron scale are easily obtainable, and so is copper scale ; zinc is 

 burnt to produce zinc white, which is used as paint ; lead is also 

 burnt on a large scale, and in this case it appears that one or 

 other of two substances is formed— litharge at a high temperature, 

 red lead at a lower temperature. This peculiarity of lead 

 suggests the study of the two products in the hope of discovering 

 the clue to a method. Weighed quantities of the litharge and 

 red lead are heated ; it is observed that only the latter changes 

 in appearance and that it loses weight. But what does it lose ? 

 It was formed by merely roasting lead in the air, and the some- 

 thing which it loses must therefore have been derived from the 

 air. If the red lead is heated in a tube a gas is given off which 

 is collected and tested — how? With a taper or glowing splinter 

 as it is to be supposed that the gas will support combustion if, as 

 is to be expected, it is the active constituent of air. The 

 discovery of the active constituent of air is thus made ! If air 

 consist of this gas and that which remains after exposing 

 phosphorus or iron in air, then by adding to such residual air as 

 much of the gas from red lead as was withdrawn, air should be 

 reobfained ; this is found to be the case. The nances of the two 



gases are now for the first time stated, and an easy method oF > 

 preparing oxygen is demon- tiated, such as that of heating 

 chlorate, but without any explanation. The conclusion pre- 

 viously arrived at, that probably in all the cases previously 

 studied of changes occurring in air, the oxygen is the active 

 substance, may now' be verified by burning or heating in oxygen 

 the substances which had been burnt in air. 



So much having been learnt of the chemistry of air, the study 

 of the pressure exercised by air may next be taken up, and the 

 common pump, the force pump, the barometer, and air currents 

 may be discussed and explained. Nowadays the charts given 

 in the daily papers, and the Ben Nevis and glycerine barometer 

 readings quoted in the Times make it particularly easy to explain 

 the barometer. The pupils should be led to make barometer 

 curves. 



Problem IV. To dcterifiine the composition of chalk. — The 

 discovery of the composition of the air in the course of experi- 

 ments made with the object of determining the nature of certain 

 changes naturally suggests that the attempt be made to ascertain 

 the composition of other things by studying the changes which 

 they undergo. Chalk is known to give lime when burnt, and' 

 experiments made in Stage III. have indicated that chalk loses 

 something when burnt — the idea that an invisible something is 

 given off is especially probable after the experiments with red 

 lead have been made ; so it is decided to heat chalk strongly, 

 but before doing this chalk and lime are examined comparatively. 

 Chalk is not observed to be altered by water ; on shaking it u]v 

 with distilled water and evaporating some of the filtered liquid 

 in a weighed dish, very little residue is obtained — so it is 

 established that it is but very slightly soluble in water. Lime is 

 slaked, weighed quantities of lime and water being used ; the 

 retention of a considerable amount of wate-, even after exposing 

 the slaked lime in a drying oven, shows that the slaking involves 

 a definite change in composition — that slaked lime is lime and. 

 water. The solubility of the lime is next determined, and found 

 to be considerably greater than that of the chalk. It is found 

 that chalk is but very slightly altered in weight when heated 

 over a gas flame, and that it is only when it is strongly heated 

 that it is converted into lime : so the chalk is strongly heated in 

 an iron tube in a Fletcher blow-pipe furnace, when gas is freely 

 given off. This is tested with a taper, which it extinguishes, so 

 it cannot be oxygen, but may be nitrogen ; if it be nitrogen, when 

 mixed with oxygen in the proportion of I to 4, it should 

 give air, but this is found not to be the case ; so evidently it is a 

 peculiar gas, and may be called chalk gas. If chalk consist of 

 this gas and lime it should be possible to reproduce chalk from 

 them ; so the gas is passed through a small weighed tube 

 containing lime, and the tube is found to get heavier. But lime 

 and chalk are so much alike that it is difficult to say that chalk 

 is formed ; perhaps dissolved lime will act similarly ; the gas is 

 therefore passed into or shaken up with lime water. The 

 precipitate which forms looks like chalk and probably is, but 

 this remains to be decided. The discovery of this behaviour of 

 chalk gas, however, is important as affording a means of again 

 comparing the gas from chalk with nitrogen. In working with 

 lime water it is scarcely possible to avoid noticing that a film 

 forms on its surface ; by exposing a quantity of the lime water a 

 considerable amount of the precipitate is obtained ; its resemblance 

 to chalk is noted, and the possible presence of chalk gas in air 

 is thus suggested ; but in view of the absence of proof of the 

 identity of the precipitates with chalk a decision is reserved. 

 The discovery is made, however, that air contains something 

 besides oxygen and nitrogen. 



It being thus establi>hed that chalk consists of two things, 

 lime and chalk gas, at this stage it is pointed out how firmly 

 these two constituents hold to each other in the chalk. The 

 absorption of the gas by the lime — its entire disappearance in 

 fact — is commented on. Accurate determinations of the loss of 

 weight on heating crystallized chalk (calc-spar) should at this 

 stage be carried out before the class, if not by the pupils, so that 

 the numbers may be quoted and that it may become impressed on 

 them that the proportions in which the lime and chalk gas are 

 present is constant. Their attention may be recalled to the 

 oxides previously studied, it being pointed out that on inspection 

 these afford no indication that they contain oxygen ; that here 

 again the gas entirely loses its individuality on entering into 

 union or combining. That oxides contain their constituents in. 

 fixed proportions may be demonstrated experimentally by 

 oxidizing finely-divided copper and determining the increase in 

 weight, lime being used as drying agent. In this way the 



