LKSSONS 



CIIKMISTUY. 



bromine, or sulphur, forming H t O, HC1, HI, HBr, an-1 H.s 

 respectively. The experiment is arranged a* in l-i/. Ut. B ia 

 u tul packed with calcium chloride, which takes all moisture 

 from the gas (this is not actually necessary), c is a "redm-tiun 

 tube."* Place in the bulb a little black oxide of copper, and 

 apply heat. The hydrogen will take the oxygen, forming H,O, 

 and the metallic copper will be found in the bulb. Thus 



CuO + H, H.O + Co. 



The same experiment may be performed with iron filings. 

 Iron has such a strong affinity for oxygen that it is never ob- 

 tained in a pure state, save by this means. The filings will 

 assume the colour of pure iron dark blue ; and when scattered 

 out of the tube into the air, so rapidly do they combine with 

 the oxygen again, that they become red-hot 



THE DIFFUSION OF OASES. 



If oil and water and mercury be shaken in a bottle for any 

 length of time, the moment the motion ceases the mercury will 

 sink to the bottom and the oil rise to the top, but this is not the 

 case with gases ; if it were, our world would not be habitable. 



Take two flasks, and connect them by a small tube passing 

 through the cork of each. Place them as in Fig. 25 ; fill the 

 top one with hydrogen, and the other with carbonic acid a very 

 heavy gas and it will be found that the 

 gases mix, the carbonic acid rising, and the 

 hydrogen falling. 



This fact may bo strikingly shown by 

 plugging up the end of a long tube with a 

 little plaster of Paris. Moisten the powder 

 into a clay, and then place some in the end 

 of the tube quickly, for it soon " sots." 

 Now fill the tube with hydrogen, and stand 

 it in a glass containing some coloured 

 water, and the water will be found to rise 

 in the tube, for the hydrogen passes out 

 through the plug with greater rapidity than 

 the air passes in ; hence the water rises. 



It has been discovered that the rate of 

 diffusion of different gases is inversely pro- 

 portional to the square root of their densities. 

 Thus oxygen is 16 times heavier than 

 hydrogen ; the square root of 16 is 4, and 

 that of 1 is 1 : therefore hydrogen passes 

 through the diaphragm 4 times faster than 

 oxygen. 



As hydrogen is the lightest of all bodies, 

 it is taken as the standard of the atomic 

 weight, and if we know its weight we can 

 always find that of any other gas ; for the 

 : densities of all these elements, which can 

 be got in the gaseous state, are identical 

 with their atomic weights. There are, how- 

 ever, two notable exceptions to this law, phosp horus and arsenic, 

 whose vapours have densities just twice their atomic weights. 



In the case of almost all compound gases, the density is half 

 the combining weight. 



The weight of a litre of hydrogen at Cent, and 760 mm. 

 [millimetres'] pressure (the standard temperature and pressure) is 

 0*08936 grammes. Therefore at the same temperature and 

 pressure 



1 litre of O weighs 16 x -08936 

 1 litre of Cl weighs 35'5 * -08936 



1 litre of CO, weighs x -08936 



I 



Ml 



1 litre of HC1 weighs _ x -08936 



Pig. 25. 



etc. 



etc. 



It is necessary to have a standard of temperature and pres- 

 sure, since the volumes of gases are very much altered by a 

 change in either. 



EFFECT OF CHANGE OF TEMPERATURE ON OASES. 



It has been found by careful experiment that when heated 

 gases expand the *}, part of their volume at for every degree 

 Centigrade, this fraction, or its equivalent decimal 0'003665, 

 is called " the co-efficient of the expansion of gases." There is 



Woulff' s bottles, 8-oz. capacity, Is. 4d. ; "reduction tube*," 6d. 



no similar co-efficient for solids, for all differ in their expan- 

 MIOIIH, whereas gases expand alike. The alteration in relume 

 for a change of temperature is easily found 



273 volumes of fas at Oat. 

 become 274 ! 



275 ., ,. 2" .. 



etc. 

 273 + 1 



etc. 

 t 



If, therefore, we require to know what volume '00 litre* of 

 hydrogen at 50 would become at 120, we ask what 273 litres 

 do under tho same circumstances. 273 would become 323 at 

 50, and 493 at 120 : hence this proportion will give as the 

 required volume: As 323 .- 493 : . 500 : the new \olume-. or 



according to the formulas V l = V f 1 + " , t ), in which V 



and V 1 are tho new and old volumes, t is the temperature of V, 

 and n tho number of degrees the temperature is altered. Foi 

 Fahrenheit degrees the expansion will be j} s X { = jj. of the 

 volume at 32 for each degree. It is right to observe that this 

 law is not absolutely accurate ; yet it is sufficiently so for all 

 practical purposes. Tho reader will note that the increment is 

 a fraction of the volume at zero, and therefore an absolute 

 quantity ; if it were a fraction of the volume at any tempera- 

 ture, the increment would bo variable. It will be evident from 

 the above that it is necessary to have a standard temperature, 

 to which to reduce all gases, in order that they may be corn* 

 pared under the same circumstances. The temperature of melt* 

 ing ice, or Centigrade, has been fixed upon 



EFFECT OF PRESSURE ON THE VOLUME OF OASES. 



Boyle, an English, and Mariotte, a French philosopher, dis- 

 covered independently of each other the law which bears their 

 name Boyle and Manotte's law " The volume of a gas ia in- 

 versely proportional to the pressure to which it is subjected." 



If, for instance, a litre of gas supports a pressure of 1 kilo- 

 gramme, and the pressure be increased to 2 kilogrammes, th* 

 volume will soon become a litre. The pressure generally 

 exerted on gases is tho weight of the atmo- 

 sphere, which is measured by the barometer. 

 If a glass tube, A B (Fig. 26), sealed at one 

 end, and about one yard long, be filled with 

 mercury, and then inverted into a vessel also 

 containing that metal, the mercury in the 

 tube will be found to fall, leaving a space, A c, 

 of about six inches empty; this is called 

 " Torricelli's vacuum.' ' That is, the column 

 of mercury in the tube is exactly the weight 

 of a column of air its own size, to the top 

 of the atmosphere. If from any cause the 

 weight of the air alter, the mercury will 

 rise or fall accordingly. The standard 

 pressure of the atmosphere is in English 30 

 inches, in French 760 millimetres. If the 

 tube were a square inch in surface, then 

 30 inches of mercury would weigh 14*67 

 Ibs., or the pressure of the atmosphere on 

 every square inch is 14*67 Ibs. When great 

 pressures are used, they are measured by 

 " atmospheres." For instance, we say a 

 gas liquefies with a pressure of 35 atmo- 

 spheres, which will equal 35 X 14'67 = 

 513*45 Ibs. on every square inch. 



According to Boyle and Mariotte' s law, 

 whenever we want to find the alteration the 

 volume of a gas undergoes by a change of 

 pressure, we form with the new pressure 

 and the standard pressure, 760 mm., a frac- 

 tion. If the pressure be increased 760 must 

 be the numerator, if diminished the denomi- 

 nator, and by this fraction we multiply the 

 original volume of the gas. For example : 

 500 litres of air are now under the pressure of 712 mm.; what 

 would be their volume at the standard pressure? Here the 

 pressure will be increased, hence the volume diminished ; the 

 fraction must therefore be leas than unity. 



500x712 

 780 ' 



-V 



