[March 1, 1893. 



KNOWLEDGE 



53 



when they finally lost their independence and joined 

 the larger body." According to Dr. Gilbert's theory the 

 moon would have been most bombarded in its equa- 

 torial regions. But there are very numerous craters in 

 the neighbourhood of the lunar poles ; indeed, the polar 

 regions seem to have been more disturbed by what is 

 usually spoken of as volcanic action than the lunar 

 equatorial regions. On the side of the moon turned 

 towards us the planes seem to be aggregated towards a 

 zone which is not greatly inclined to the plane of the 

 lunar orbit. 



A novel idea was brought out by Mr. Francis Galton 

 in a lecture delivered at the Royal Institution on .January 

 17th. It consists in expressing outline drawings by means 

 of letters or numbers. Anyone can understand that 

 sketches of a face, or an outline map, can be reproduced 

 by arranging a number of small discs or counters so as to 

 imitate the alignment of figures and that any figure could 

 be constructed, step by step, if the bearing of each disc, 

 with respect to the one before it, is known. Suppose each 

 of the sixteen principal points of the compass is represented 

 by a separate letter of the alphabet, and it were desired to 

 translate a map of England into a formula on Mr. Galton's 

 plan. The outline would be imitated by laying down a 

 series of small discs on a flat surface. Then, beginning, 

 say from Land's End, the bearing of such disc with respect 

 to the preceding one would be observed, and the letter 

 which distinguishes the bearing, would be written down. 

 When this has been done for the entire map, a string of 

 letters is obtained as its formula. The map could after- 

 wards be reconstructed from this formula at any place by 

 laying down discs or points in the manner indicated by the 

 letters. The utility of translating a drawing into a 

 formula, and then reconstructing the same drawing from 

 it, is not obvious at first sight ; but when it is remembered 

 that such a formula could be transmitted by telegraph, the 

 importance of the plan becomes apparent. The site of a 

 battle, the plan of a newly-explored region, a design, or 

 even a portrait, could be wired as easily as any other 

 information, and the cost need not be great. Mr. Galton 

 exhibited a map of England at the lecture made with 218 

 dots, and its cost of transmission from the United States 

 would only be about £5, which is small in comparison with 

 theamountsspent by newspapers on telegraphic information. 

 It seems very probable, therefore, that pictorial telegraphy 

 " may have a future." 



THE CONSTITUTION OF GASES. 



By -T. .1. Stewart, I'/' /■jiiiikiiiucI ( 'ollri/i', Wnnliriiliii . 



WITHIN comparatively recent years, it was 

 generally taught that the pressure exerted by 

 a gas on the sides of the vessel which con- 

 tained it was due to a mutual repulsion of 

 the particles of the gas. Now, the beautiful 

 kinetic theory of gases furnishes us with a much more 

 satisfactory and withal more simple explanation of the 

 behaviour of gases. While this hypothesis, in its main 

 features, is simple, and easily understood, to advance any 

 distance into its method and results requires considerable 

 mathematical knowledge. Still, as the statement of the 

 principal laws of matter in the gaseous state, in accordance 

 with the kinetic theory, can be readily made and grasped, 

 I propose to endeavour shortly to give the results hitherto 

 arrived at by scientific men working in this department 

 of physics, together perhaps with some of their methods 

 of investigation, so far as the latter can be done without 

 the aid of mathematical symbols. 



The anticipations of modern discovery which were made 

 by some of the keener intellects, among the ancient Greeks 

 especially, is most remarkable. The idea that many of 

 the observed properties of bodies around us, which are 

 apparently at rest, are due to the particles composing these 

 bodies behig in reality in a state of rapid motion amongst 

 themselves, is to be found distinctly stated by the Roman 

 writer Lucretius. His conception of atoms " strong in 

 sjlid singleness " comes surprisingly near to some modem 

 hypotheses as to the constitution of matter. 



The atomic theory of matter was expounded by 

 Democritus of Abdera, in Thrace, and the interest felt at 

 that time, /.c, about 450 b.c, in such questions is shown 

 by the fact that the lecturer realized a handsome sum in 

 talents of gold. 



Auaxagoras, known as the teacher of Socrates, stated a 

 theory which was diametrically opposed to that of 

 Democritus. According to his speculations, matter is 

 infinitely divisible; the smallest divided portions can again 

 be divided, and this without end. Thus he considered 

 matter to be homogeneous, to have the same structure 

 throughout, and that there was no limit to the smallness 

 of the possible particles. 



On the other hand, Democritus taught that matter is not 

 infinitely divisible, but that if we could continually divide 

 up portions of matter we would at length reach particles 

 which could no further be divided, i.e. atoms — things 

 which cannot be cut. In this belief Democritus was 

 followed by Epicurus and Lucretius, and it is to a very 

 similar idea that modern research has been led. But the 

 present views of physicists are founded on the method of 

 questioning Nature by experiment, and reasoning on the 

 resiUts found in answer to their enquiries. These ancient 

 philosophers spun their more or less ingenious theories out 

 of a priori notions in their own minds. 



Daniel Bernoulli, the mathematician, born in 1700, was 

 probably the originator of the thought that the pressure 

 exerted by a gas on the walls of its containing vessel is 

 caused by the bombardment of these walls by the minute 

 ])articles of matter making up the gas. For long after his 

 time no further step was made in discovery, and his 

 remark seems to have remained comparatively unnoticed. 



Roughly stated, the kinetic theory of gases may be said 

 to be this : A gas consists of separate particles or molecules 

 of matter at a distance from each other, and of excessive 

 smallness ; these are continually flying about in all 

 directions, and in their passage they strike against each 

 other, and also against surrounding bodies. Imagine a 

 volume of gas shut into a glass vessel, then the sides of 

 this vessel are exposed to a continual bombardment from the 

 particles of the gas dashing against them. This bombard- 

 ment of molecules is so continuous and rapid, and the 

 number of particles which strike against a given portion of 

 the containing walls is so large, that the result is a pressure 

 outwards on the vessel caused by the gas, and this being 

 resisted by the rigidity of the glass, an equal and opposite 

 pressure inwards balances the pressure outwards exerted 

 by the gas. Though we can never hope to capture or to 

 see a single molecule, yet man}- experiments and 

 considei'ations, converging from different starting-points, 

 prove that something of this sort goes on in all gaseous 

 bodies. Of course the assumption of a hard solid atom 

 is entirely arbitrary, and can only be considered a first 

 approximation to the actual truth ; yet many of the 

 properties of matter can be explained on the hypothesis 

 that it is made up of such hard spheres, and thus this 

 conception is a valuable aid to fixing and making definite 

 our views as to the constitution of matter. It also serves 

 to express concisely the results arrived at by experimental 



