444 



NA TURE 



\_March lo, 1881 



that under such electrical influence the falling mercury may be 

 able to decompose aqueous vapour at these high exhaustions, 

 with formation of oxide of mercury and liberation of hydro- 

 gen. Of these two theories the latter appears to be the more 

 probable. 



The presence of water vapour shows itself likewise in the 

 very slight amount of repulsion produced by radiation. Re- 

 pulsion commences in air at a jMessure of 12 millims., whilst 

 at a higher exhaustion the maximum effect rises to over 40 divi- 

 sions. Here, however, repulsion does not begin till the 

 exhaustion is higher than the barometer gauge will indicate, 

 whilst the maximum action after long-continued pumping is only 9 

 divisions. 



Viscosity of Kertsoline Vapour, — The rapid diminution of 

 viscosity in the last experiment after reaching the pressure of 

 400 millims. , is probably due to the aqueous vapour in the air 

 being near its liquefying point. It w as thought advisable to test 

 this hypothesis by employing a somewhat less easily coudensible 

 vapour, which could be introduced into the apparatus without 

 any admixture of air. An experiment was accordingly tried 

 with a very volatile hydrocarbon, commercially known as Kero- 

 soline, boiling at a little above the ordinary temperature. The 

 vapour of this body was introduced into the well-exhausted 

 apparatus, when the gauge at once sank 82 '5 millims. After 

 the usual precautious to eliminate air a series of observations 

 were taken. 



The lois of viscosity is more rapid than with any other gas 

 examined except aqueous vapour. Conversely a very great in- 

 crease of viscosity occurs on increasing the pressure from 8 to 

 82*5 millims. The explanation of this is that the vapour of keroso- 

 line is very near its liquefying point, and therefore very far from 

 the state of a " perfect " gas. 



The negative bend in the curve at about 10 millims. pressure, 

 already noticed with other gases, is strongly marked with this 

 hydrocarbon vapour. 



JDiscussioH of Results. — When discussing the viscosity results 

 obtained with the different gases experimented with, the author 

 gives the following approximate comparison of viscosities, such 

 as is afforded by a comparison of the log decs, of each gas and 

 that of air, comparing the ratio with that obtained by Graham, 

 Kundt and Warburg, and Maxwell. 



Cundt & 



Maxwell. Crookes. 

 I '0000 I '0000 



— i'ii85 



— 0-9715 



— o'97i5 

 Carbonic anhydride .. o'So; o'8o6 o'859 o'92oi 

 Hydrogen o"4S55 o'4SS o*5is6 o'4439 



Graham's numbers are the theorttical results deduced from his 

 experiments on transpiration of gases. They are, he says,^ the 

 numbers to which the transpiration times of the gases approxi- 

 mate, and in which they have their limit. Graham concludes 

 that the "times of oxygen, nitrogen, carbonic oxide, and air are 

 directly as their densities, or equal weights of these gases pass 

 in equal times. Hydrogen passes in half the time of nitrogen, or 

 twice as rapidly for equal volumes. The result for carbonic acid 

 appears at first anomalous. It is that the transpiration time of 

 this gas is inversely proportional to its density when compared 

 with oxygen." 



The proportion between air and oxygen, nitrogen, or carbonic 

 oxide is not very different at any degree of exhaustion to that 

 which it is at 760 millims. Carbonic anhydride, however, is 

 different ; the proportion between it and air holds good be- 

 tween 760 and 650 millims. Then it gets lower and lower as 

 the pre-sure sink ■, until 50 or 55 millims is reached, when the 

 proportion between it and air again becomes constant. 



Hydrogen, however, is entirely different to the other gases ; 

 its log dec. remains the same to a very high exhaustion, and, that 

 of other gases sinking, it is evident tliat the proportion between 

 this gas and any other is different for each pressure. 



It must not be forgotten that the pressure of 760 millims. is 

 not one of the constants of Nature, but is a purely arbitrary one, 

 selected for our own convenience when working near the level of 

 the sea. In the diagrams accompanying his R.S. paper the author 

 has started from this pressure of 760 millims., and has given the 

 log dec. curves which approximately represent the viscosities 

 through a wide range of exhaustion. But the curves might also 

 be continued, working downwards instead of upwards. From 

 I Loc, cit. pp. J78, 179. 



the shape and direction in which they cut the 760 line, it is 

 reasonable to infer their further progress downwards, and we 

 may assume that an easily liquefiable gas will show a more rapid 

 increase in viscosity than one which is difficult to liquefy by pres- 

 sure. For instance, hydrogen, the least condensible of all gases, 

 shows scarcely any tendency to increase in log dec by pressure. 

 Oxygen and nitrogen, which are only a little less difficult to con- 

 dense than hydrogen, show a slight increase in log dec. Carbonic 

 anhydride, which liquefies at a pressure of 56 atmospheres at 

 15° C, increases so rapidly in log dec. that at this pressure it 

 would have a log dec. of about i -3, representing an amount of 

 resistance to motion that it is difficult to conceive anything of 

 the nature of gas being capable of exerting. 



Kerosoline vapour is rendered liquid by jiressure much more 

 readily than carbonic anhydride. Its curve shows a great 

 increase in density for a very slight access of ]iressure. 



Again, aqueous vapour is condensible to the liquid form with 

 the greatest readiness ; and the almost horizontal dhection of the 

 curve representing aqueous vapour mixed with air carries out the 

 hypothesis. 



It follows, then, that Maxwell's law holds good for perfect 

 gases. The disturbing influence spoken of in the commencement 

 of this paper as occasioning a variation from Maxwell's law, is 

 the tendency to liquefaction, which prevents us from speaking of 

 any gas as " perfect," and which hinders it from obeying Boyle 

 and Mariotte's law. The nearer a gas obeys this law the more 

 closely does it conform to Maxwell's law. 



Maxwell's law was discovered as the consequence of a mathe- 

 matical theory. It pre.'upposes the existence of gas in a "per- 

 fect " state — a state practically unknown to physicists, although 

 hydrogen gas very nearly approaches that state. An ordinary 

 gas may be said to be bounded, as regards its physical state, on 

 the one side by the sub-gaseous or liquid condition, and on the 

 other side by the ultra-gaseous condition. A gas assumes the 

 former state when condensed by pressure or cold, and it changes 

 to the latter state when highly rarefied. Before actually assum- 

 ing either of these states there is a kind of foreshadowing of 

 change, with partial loss of gaseity. When the molecules, by 

 ])ressure or cold, are made to approach each other more closely, 

 they begin to enter the sphere of each other's attraction, and 

 therefore the amount of pressure or cold necessary to produce a 

 certain density is less than the theoretic d amount by the internal 

 attraction exerted on each other by the molecules. The nearer 

 the gas approaches the point of liquefaction the greater is the 

 attraction of one molecule to another, and the amount of 

 pressure required to produce any given density will be pro- 

 portionally less than that theoretically required by a " perfect" 

 gas. 



A noteworthy point in connection with the elasticity of glass 

 is ob-erved on the curves of viscosity. They are not continued 

 beyond the 0-02 M exhaustion, but the general form of the curves 

 indicates that, if they were produced beyond the limits of the 

 observations, they would cut the line representing the absolute 

 vacuum. The curve representing the repulsion accompanying 

 radiation evidently goes up to the zero point, showing that; at 

 an absolute vacuum there would be no repulsion. The curves of 

 viscosity cannot, however, be supposed to end at the z-:ro point 

 without a sudden change in direction. They evidently touch the 

 top line of zero pressure long before the log dec. of 0-00 is 

 reached. This means that in an absolute vacuum there would 

 still be a measurable amount of viscosity. This is probably due 

 to the viscosity of the glas torsion fibre, for it has been ascer- 

 tained that glass is not perfectly elastic, but will take a permanent 

 set if kept under constraint for a considerable time. 



The author gives an instance which has come under his own 

 notice. In 1S62 he purchased a piece of glass lace, and some 

 spun glass from which the lace was made. The spun glass is in 

 long straight threads, about Q-ooi inch diameter, and has occa- 

 sionally been used for torsion fibres. The fibres of which the 

 lace was made were origi nally straight, but the twists and bends 

 in w hich they have been kept for eighteen years have permanently 

 altered their direction, .and on dissecting a portion of the lace the 

 component fibres remain distorted and bent, even when free to 

 resume their original shape. 



We.-e glass perfectly elastic the log dec iu an absolute vacuum 

 would probably be equal to zero : there would then be no dimi- 

 nution in the arc of vibration, and the torsion fibre once set 

 swinging would go on for ever. 



T/ie Ultra- Gaseous State of Matter.— K consideration of the 

 curves of viscosity of the gases, especially hydrogen, which are 



