176 



KNOWLEDGE. 



[September 1, 1892. 



us by fossil teeth from the very lowest secondary strata 

 of Europe, was actually represented by one apparently 

 belonging to the same genus in the rivers of 

 Australia. It is true, indeed, that a recent discovery 

 has shown us that the living baramunda differs 

 slightly in the structure of its skull fi'om the fossil 

 Ciratodus, and that the teeth of the opposite sides of the 

 jaws were not in actual contact with one another, as were 

 those of the latter. Although these points of difference are 

 considered sufficient to warrant us in regarding the 

 baramunda as not actually belonging to the same genus 

 as the fossil teeth, yet this does not detract from the 

 extreme interest of this fish as being by far the oldest 

 type of back-boned animal now living. This type of fish 

 is, indeed, thus proved to have endured throughout the 

 whole of the immense period during which the entire 

 series of secondary and tertiary rocks were deposited ; 

 and when we reflect that the secondary rocks include 

 those enormous accumulations of strata known as the trias, 

 lias, oolites, greensands, and chalk, while the tertiary com- 

 prises the threefold divisions termed eocene, miocene, and 

 pliocene, we can scarcely fail to be almost lost in wonder 

 at the prodigious length of time during which baramundas 

 have existed, with but comparatively slight structural 

 modification. The fossil baramimdas occur in the 

 secondary rocks of Europe, Africa, India, and North 

 America, and it is an interesting subject of speculation 

 why the group should have totally disappeared from all 

 those regions, to find a last home in far Australia. 



The existing baramunda lives mainly or entirely on 

 leaves, and we may therefore conclude that the fossil teeth 

 hkewise pertained to fishes which subsisted on somewhat 

 similar nutriment. If, however, we were to infer that all 

 the teeth of fossil fishes which have a ridged or flattened 

 grinding surface belonged to herbivorous types, we should 

 be sadly in error, since many of them, approximating more 

 or less markedly to the Cenitodiis type, really indicate fishes 

 allied to the well-known Port Jackson shark, in which the 

 mouth is covered with a complete pavement of flattened 

 teeth adapted for crushing sheU-fish and other hard animal 

 substances. In all such investigations, the truth can, 

 indeed, only be found out by careful induction, and by 

 availing ourselves of every scrap of information left to us 

 among the relics of former epochs. 



RADIANT MATTER. 



By A. Jameson. 



AN article on " Kadiometry," that ajipeared in a recent 

 issue of Knowledge, shortly described how some 

 of the energy of ether waves, striking an absorbent 

 surface, and being transformed into heat — that is, 

 into disturbance of the molecules of matter — • 

 may give rise to what is known as Crookes' pressure, and 

 to mechanical motion. It was explained that light mills, 

 as ordinarily made, will only operate when surrounded by 

 a gas in " the fourth condition," i.e., where the free paths 

 of the molecules were long compared with the diameter of the 

 light mill. It is now proposed to relate briefly some of 

 the other experimental facts through which the discovery 

 of this condition of matter has so much improved our 

 knowledge of the actual condition in which gaseous matter 

 exists — that is, of the way in which the molecules of gases 

 are continually moving. 



It is perhaps unnecessary to preface the remark that 

 this nominal distinction between gaseous and ultra- 

 gaseous matter is, although a useful, a purely arbitrary 

 one, and that the two conditions are really in the strictest 



sense of the word continuous ; for the accepted definition 

 of a gas is that it is matter, any portion of which is 

 capable of expanding indefinitely, so as to exert a pressure 

 upon the walls of any containing vessel, however large. 

 Not that even this broad definition is altogether unex- 

 ceptional. It might be said, for instance, to be inapplicable 

 in the case of those gaseous masses that were described 

 in Mr. Eanyard's article last July, and the extreme 

 limits of which could probably be calculated in the 

 same way and with the same degree of precision as 

 that of the earth's atmosphere was calculated in 

 Knowxedge for November last. But this point is only 

 mentioned incidentally ; we are not much concerned with 

 it at present. Gas molecules at a given temperature, or 

 that are moving relatively to one another at a given average 

 speed, are, one might suppose, unaffected individually by 

 the pressure of the gas of which they are constituents. 

 But there are slight exceptions to be taken to this statement 

 also. We have, for instance, the mutual actions (attractions, 

 and perhaps some additional inter-molecular action of 

 adjacent molecules) making themselves felt in proportion 

 to the square of the density, and constituting the well- 

 known deviation from Boyle's law.'' Another distinction 

 that will be mentioned is of greater interest in connection 

 with the subject of high exhaustion. It must be premised 

 that molecules themselves are elastic bodies whose consti- 

 tuent parts are capable of oscillatory motions (intra-mole- 

 cular vibrations). These relative motions of the parts of 

 individual molecules are excited by transverse ether waves, 

 and by molecular impacts. The average velocity of trans- 

 lation of the molecules of a gas represents its sensible heat ; 

 while in addition to this we have the energy concerned in 

 the pulsations of individual molecules or chemical atoms. 

 The intra-molecular vibrations, excited as has been 

 described, are capable in their turn of causing loco- 

 motion of the molecule, by virtue of the reaction 

 that takes place when this pulsating body comes in 

 contact with any resisting body. They also give 

 rise to radiations of transverse waves in the ether, and 

 in fact Prevost's law of exchanges of radiant heat holds as 

 perfectly between the molecules of a gas as in any other 

 circumstance. Each molecule is continually throwing oft 

 ether waves at the expense of its intra-molecular energy, 

 and is continually deriving reinforcement of that energy 

 from the ethereal radiations of neighbouring molecules, or 

 of surrounding substances. Now in the case, for example, 

 of a gas undergoing the process of cooling, the oscilla- 

 tions of the chemical atoms die away to some extent 

 between successive encounters, owing to the radiation of 

 their energy. Just asavibrating bell throws off aerial waves, 

 so a vibrating molecule generates waves of a somewhat 

 dift'erent character in the medium with which it is sur- 

 rounded ; and if the material vibrations excited by striking 

 the elastic body are renewed less frequently, the result, 

 similar in each case, is a diminution of the average intensity 

 of those oscillations. Hence very highly rarefied gases, in 

 which the interval of time between successive collisions is 

 considerable, I in which, that is to say, the mean free 

 path is greatly extended, must cool more slowly by radia- 

 tion than would be the case were they in a condition of 

 greater density. Such distinctions as these between the 

 gaseous and ultra-gaseous states serve, however, to accen- 



* Boyle's law, as the reader is doubtless aware, asserts that the 

 pressure and volume of a gas are inversely proportional to one 

 another. 



f This interval in hydrogen at the ordinary temperature and 

 pressure is, according to Professor Tait, only one 17,700,Oi)0,OOOth part 

 of a second, that is to say, each particle of hydrogen has the direction 

 of its motion changed on an average 17,700,000,000 times per second. 



