88 



KNO^A^LEDGE 



[Dbc. 2, 1881. 



tlifii, is the essential diflerenee lietwoeu liiiuid fluidity and 

 , iseous fluidity '? Tlie expei-t in molecular mathematics, 

 'iscoursin;; ti) his kinematical l)rcthren, would produce a 

 tivmeiidous reply to this tjuestion. lie would descriljc the 

 .iscillations, gyrations, collisions, mean free paths, and 

 mutual olistructions of atoms and molecules, and, by the aid 

 <if a maddening aiTay of symbols, arrive at the concluaion 

 that gases, unless restrained, are liable to indefinite or vast 

 ■ xpansion, while liciuids, of their own accord, retain definite 

 limits or dimensions. 



The mattor-of-fact experimentalist demonstrates the same 

 by methods that are easily understood by anybody. I shall, 

 therefore, both for my own sake and my reader's, describe 

 some of the latter. 



In the first place, we all see plainly that liquids have a 

 surface, i.r,., a well-defined boundary, and also that gases, 

 iiidess enclosed, have not. But as this may be due to the 

 invisibility of the gas, we must question it further. The 

 air we breatlic may be taken as a type of gases, as water 

 may of li(iuids. It has weight, as we may prove by weigh- 

 ing a bottle full of air, then pumping out the contents, 

 weighing the empty bottle, and noting the difference. 



Having weight, it presses towards the earth, and is 

 squeezed by all that rests above it, and thus the air 

 around us is constrained air. It is very compressible, 

 and is accordingly compressed by the weight of all the 

 air above it. 



This being understood, let us take a bottle full of water 

 and another full of air, and carry them both to the summit 

 of Mont Blanc, or to a similar height in a balloon. We 

 shall then have left nearly half of the atmosphere below, 

 and thus both liquid and gas will be under little more than 

 half of the ordinary pressure. What will happen if we 

 uncork them both 1 The liquid will still display its definite 

 surface, and remain in the bottle, but not so the gas. It will 

 overflow upwards, downwards, or sideways, no matter how 

 the bottle is held, and if we had tied an empty bladder o^ er 

 the neck before uncorking, we should find this overflow or 

 expansion of the gas exactly proportionate to the removal 

 of pressure, provided the temperature remained unalteied. 

 Thus, at just half the pressure under which a pint bottle 

 was corked, the air would measure exactly one quart, at 

 one-eighth of the pressure one gallon, itc. 



We cannot get high enough for the latter expansion, but 

 can easily imitate the effect of further elevation by means 

 of an air-pump. Thus, we may put one cubic inch of air 

 into a bladder of 100 cubic inches capacity, then place this 

 under the receiver of an air-pump, and reduce the pressure 

 outside the bladder to y^u*'' ^^ its original force. With 

 such atmospheric surrounding, the one cubic inch of air 

 will plump out the flaccid bladder, and completely fill it. 

 The pumpability of the air from the receiver shows tliat it 

 goes on overflowing from it into the piston of the pump as 

 fast as its o\vn elastic pressure on itself is diminished. 



Numberless other experiments may be made, all proving 

 that all gases are composed of matter which is not merely 

 incohesive, but is energetically self-repulsive ; so much so, 

 that it can only be retuincd within a.v\ bounds whatever by 

 means of some exti'rnal pressure or constraint For aught 

 we know experiineutttlly, the gaseous contents of one of 

 Mr. Glaisher's balloons would outstretch itself suftlciently 

 to occupy the whole sphere of space that is spanned by the 

 earth's orliit, provided that spac(! were perfectly vacuous, 

 and the l)alloon were burst in the mid.st of it, and the 

 temperature of the expanding gas were maintained. 



Here, then, in this self-repulsiveness, instead of self- 

 cohesion, this absence of self-imposed boundary or dimen- 

 sions, we ha^e a very broad and well-marked distinction 

 between gases and liquids, so I'road that there seems no 



bridge that can possibly cross it. Tliis was Itelieved to be 

 the case until recently. Such a bndge has, however, Ijcen 

 built, and rendered visible, by tlie experimental researches 

 of Dr. Andrews; Imt further explanation is required to 

 render this generally intelligible. 



Until qiiite lately it was customary to divide gases into 

 two classes — "pennanent gases" and "condensable gases" 

 or "vapour.s." Ga.seous water or steam wa.s usually de- 

 scribed as typical of the latter ; oxygen, hydrogen, or 

 nitrogen of the former. Earlier than this, many other 

 gases were included in the [lermanent list ; but Faraday 

 made a serious inroad upon this classification when he 

 liquefied chlorine by cooling and compressing it. Long 

 after this, the gaseous elenients of water, and the chief 

 constituents of air, oxygen, liydrogen, and nitrogen, re- 

 sisted all efforts to condense them ; but now they have 

 succumbed to great pressure and extreme cooling. 



We thus arrive at a very broad generali.sation, viz., that 

 all gases are physically similar to steam, (I mean, of course, 

 "dry steam,' i.e., true invisible steam, and not the cloudy 

 matter to which the name of steam is popularly given,) 

 that they are all formed by raising liquids above their 

 boiling-point, just as steam is formed when we boil water 

 and maintain the steam above the boiling-poiiit of the 

 water. 



But some liquids boil at temperatures far below that at 

 which others freeze ; liquid chlorine boils at a temperature 

 below that of freezing water, and liquid carljonic acid below 

 even that of freezing mercury, and liquid hydrogen far 

 lower still. These are cases of boiling, nevertheless, though 

 it seems a paradox according to the ideas we commonly 

 attach to this word. But such ideas are based on our 

 common e.xperience of the properties of our commonest of 

 liquids, viz., water. 



When water boils imder the conditions of our ordinary 

 experience, the passage from the liquid to the gaseous state 

 is a sudden leap, with no intermediate state of existence 

 that we are able to perceive ; and the conditions upon 

 which water is converted into steam — the liquid into the 

 gas — while both are at the bottom of our atmospheric 

 ocean, are such as to render an intermediate condition 

 rationally, as well as practically, impossible. 



We find that the expansi\c energy by which the steam is 

 enaViled to resist atmospheric pressure is conferred upon it 

 by its taking into itself, and utilising for its expansive 

 efforts, a large amount of calorific energy, ^^^len any given 

 quantity of water is converted into steam, under ordinary 

 circumstances its bulk siidden/i/ becomes above 1,700 times 

 greater — a cubic inch of water forms about a cubic foot of 

 steam, and nearly 1,000 degrees of heat (9GG-6) disappears 

 as temperature. Otherwise stated, we must give to the 

 cubic inch of water at 212° as much heat as would raise it 

 to a temperature of 212 plus 96G-6, or 1178-G° if it 

 remained liqiud. Tliis is about the temperature of 

 the glowing coals of a common tire ; but the steam 

 that has thus taken enough heat to make the water red 

 hot is still at 212° — no hotUr than the water was wliile 

 boiling. 



This heat, which thus ceases to exhibit itself as tempera- 

 ture, is otherwise occupied. Its energy is partly devoted to 

 the work of increasing the bulk of the water to the above- 

 named extent, and partly in confeiring on the steam its 

 gaseous speciality — that is, in overcoming liquid cohesion, 

 and substituting for it tlie opposite property of internal 

 repulsive energy which is charact<n'istic of gases. My 

 reasons for thus defining and sejiai-ating these two functions 

 of the so-called " latent " heat will be seen in the next 

 paper, when we come to the philosophy of the interesting 

 researcl'.es of Dr. Andrews. 



