236 REPORT — 1889. 



realised 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 phosphorus 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 se]parate the active from the inactive constituent 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 something 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 with- 

 drawn, air should be re-obtained ; this is found to be the case. The names 

 of the two gases are now for the first time stated, and an easy method of 

 preparing oxygen is demonstrated, such as that of heating a chlorate, but 

 without any explanation. The conclusion previously arrived at, that 

 probably in all the cases previously studied of cha,nges 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. The com- 

 parison of the densities of the two gases with that of air should then be 

 made. 



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 Glycerin barometer readings quoted in the ' Times ' make it 

 particularly easy to explain the barometer. The pupils should be led to 

 make baT'ometer curves. 



Problem IV. To determine what happens luhen chalk is hurnt to lime. 

 — The discoveiy of the composition of the air in the course of experiments 

 made with the object of determining the nature of certain changes natu- 

 rally suggests tbat the attempt should 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. 



