THE POPULAR EDUCATOFv. 



end of the spring may be drawn down, and its power thus 

 altered, so as to give the best action to the lever. 



If the lamp be blown out, or in any way extinguished, the 

 spring immediately draws down the end L of the lever, and 

 thus lifts the lower pole till it touches the upper, so that the 

 re-lighting is practically instantaneous. In fact, it will re-light 

 itself very many times in the course of a minute. The working 

 parts, too, are so simple that when carefully made they cannot 

 well get out of order. 



Frequently it is required to introduce the vapours of various 

 metals into the electric light. In this case the lower pole is 

 made of a rod of graphite about half an inch in diameter, and 

 is slightly hollowed at the top so as to form a small crucible, 

 into which a fragment of any metal, or a salt of it, may be intro- 

 duced. Experiments of this kind have been much tried lately 

 for the purposes of spectrum analysis. By means of a lens 

 and a series of prisms, the ray from the electric lamp is opened 

 out into a spectrum which contains all the colours of the rain- 

 bow, and it has been found that if any metal or metallic salt be 

 placed on the poles, certain lines or bands will be seen across 

 the spectrum, and these will show distinctly what metal has 

 been employed. So delicate is this mode of testing, that several 

 new elements have already been discovered by it. Many great 

 discoveries in physics have also been made in the same way, for 

 not only can an artificial ray of light be thus analysed, but the 

 light of the sun and stars has been tested, and the presence of 

 various metals in these bodies has been distinctly proved. 



PNEUMATICS. I. 



OBJECTS OF THE SCIENCE PROPERTIES OF THE AIR ITS 



WEIGHT DIVING-BELL AIR-PUMP. 



IN our first lesson on Hydrostatics we saw that all bodies were 

 divided into three great classes solids, liquids, and gases 

 according to the relations subsisting between their ultimate par- 

 ticles and the relative distances at which they are placed from 

 one another. Of the properties of the first and second of these 

 classes we have treated in our lessons on Mechanics and Hydro- 

 statics ; and the science which is now to engage our attention 

 is concerned with the motions, pressure, weight, etc., of the 

 third. The term " Pneumatics " is derived from the Greek word 

 pneuma, which signifies "breath" or "air," and it therefore 

 means the science which treats of air ; not that it is occupied 

 exclusively with air, but as in Hydrostatics water is taken as a 

 type of all liquids, so here air is taken as a type of all gases, 

 being the most familiar of them all. There are many different 

 gases, but in their physical properties they, for the most part, 

 very closely resemble common air ; and the points of difference 

 in their composition and chemical properties it is not our pro- 

 vince to treat of here. When, therefore, in these lessons we 

 speak of air, it should be remembered that the results obtained 

 are. with necessary modifications, true of other gases. 



In many of their properties gases are very closely allied to 

 liquids, hence many of the principles we arrived at in Hydro- 

 statics, as relating to liquids, apply equally to gases their 

 particles move over one another with scarcely any friction, and 

 they transmit pressure equally in all directions. 



.There is, however, this great difference, that the ultimate 

 particles of any liquid have a certain amount of attraction for 

 each other, while those of a gas repel one another ; and thus if 

 the space in which it is enclosed be enlarged, it will at once ex- 

 pand and completely fill it. If a liquid is contained in a vessel, 

 the pressure it exerts upon the sides results solely from its 

 weight ; but when a gas is thus confined there is, in addition to 

 this, a pressure on all parts of the containing surface, arising from 

 the elastic pressure of the gas itself. A gas, too, is highly in 

 fact almost indefinitely compressible, while, as we have' seen, 

 for all practical purposes, a liquid is absolutely incompressible. 



Now as air is by far the most important of all pases, we 

 shall inquire a little into its properties before considering gases 

 generally. The atmosphere, then, is a layer of air completely sur- 

 rounding the earth on all sides, and extending upwards to a height 

 usually computed at about forty-five or fifty miles. It fills every 

 space on the earth's surface, and presses, as we shall see, on all 

 bodies with an immense force. We are completely surrounded 

 by it ; we live, in fact, at the bottom of an immense ocean of it ; 



and yet, except when it is put in motion, we scarcely notice its 

 presence. Though thus unnoticed, however, it is of the utmost 

 importance to us. Without it all life, animal or vegetable, 

 would droop and die ; our fires and lamps would refuse to burn ; 

 and when the sun shone, instead of even gradations of light and 

 shade, we should have either almost intolerable brightness or the 

 blackest darkness. No clouds would shade the sun, nor any rain 

 fall to water the earth ; all would be a barren, lifeless blank. We 

 see, then, something of the benefits we derive from it, and these 

 surely render it desirable for us to study some of its phenomena. 

 Its chemical properties have already been explained in our lessons 

 on Chemistry; we need, therefore, ' say little about them. It is 

 not a simple gas, but a mixture consisting almost entirely of 

 oxygen and nitrogen, in the proportion of nearly 21 parts by 

 volume to 79 of the latter. Small quantities of carbonic acid 

 and watery vapours are also present. The former of these is a 

 poisonous gas given off in the breath, and by fires, and burning 

 bodies, and would speedily accumulate, so as to destroy life, had 

 not the Creator mercifully caused that trees should feed upon 

 it, removing the carbon it contains, and building that into their 

 own structures, while they set free again the oxygen which 

 was united with it in the gas. Winds mix the different portions 

 of the air, and thua remove this gas from crowded cities and 

 bring in its place purer air. 



The watery vapour in the air varies very greatly in amount, 

 and, as will appear, performs a very important office, being the 

 cause of rain and dew. 



Though we notice the presence of the air so little, it is a 

 material substance ; that is, it occupies space to the exclusion 

 of other bodies. 



A simple experiment will furnish conclusive proof of this. 

 Float a cork on a vessel of water, and invert over it a glass 

 jar. On pressing the jar down, the position of the cork will 

 show that the level of the water inside is below that outside ; 

 something, then, must be there to press it down, and that 

 something is the air contained in the jar. If we have a stop- 

 cock inserted in the top of the jar, or use a bottle with the 

 bottom cut off, on opening the mouth the air will rush out, and, 

 the pressure being removed, the water inside will rise to its 

 former level. We see, then, that though the jar would have 

 been stated to be empty, it was in reality full of air. In the 

 same way a bladder or air-cushion may be filled with air, and 

 will sustain pressure almost as if it were solid. 



If we take a large sheet of thick cardboard, or an open 

 umbrella, and run, holding it so that the air meets its flat 

 surface, the resistance we shall experience '"ill afford an 

 additional proof that the air which thus opposes the motion is 

 really a material substance. 



The experiment we mentioned above namely, immersing a 

 glass jar in water though so simple, is an important one, as it 

 illustrates to us the principle of the diving-bell. In laying the 

 foundations of bridges, piers, or other structures rising out of 

 water, it is very desirable, and, in fact, absolutely necessary at 

 times for some person to be down at the place where the work 

 is going on. Now if a coffer-dam had to be constructed to 

 keep out the water it would add greatly to the expense of the 

 work, and also to the time occupied, but this can be dispensed 

 with by the use of a diving-bell. This consists merely of a large 

 iron vessel, made strong enough to resist the pressure of the 

 water. It is open at the bottom, and has a ledge round it, on 

 which people may sit. The bell is raised a little above the 

 level of the water, so that the workpeople may enter it, and 

 then it is gradually lowered into the water by means of chains ; 

 the air inside keeps out the water, so that those within remain 

 dry. As the bell descends, however, the air becomes compressed, 

 and the water rises a little way. To remedy this, and also to 

 maintain a supply of pure air, pipes are brought down from 

 some powerful force-pumps, and by means of these the bell is 

 kept full, and supplied with fresh air. Thick glass windows 

 are placed in the top to give light to those within. The condensa- 

 tion of the air by the pressure of the water produces a sense of 

 oppression, and frequently a pain in the eyes or ears : this, how- 

 ever, gradually passes away. The men are sometimes provided 

 with a waterproof dress and helmet, clothed in which they can 

 get out of the bell, and walk about at the bottom, air being con- 

 veyed to them by pipes. Frequently, indeed, the bell is dispensed 

 with altogether, and these dresses only used; the air-pipe opening 

 into the helmet, and the excess and waste air escaping by a suit- 



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