Sept. 6, 1888] 1 



NATURE 



461 



will not separate again, except under special circumstances ; but 

 so far the fact that different gases behave differently in this 

 respect remains unexplained. If two spherical bodies collide, 

 they will remain in contact only if perfectly inelastic, otherwise 

 they will fly off in opposite directions. 



In the latter case the elastic forces are due to the displacement 

 of the molecules of the spheres from their positions of equi- 

 librium. If the colliding bodies are two of Thomson's atoms, 

 similar elastic forces will be called into play by a displacement 

 of their outer shells. If the mass m x of each of the outer shells 

 is very large compared with that of the inner ones, the outer shells 

 will remain nearly at rest after the collision, while the inner ones 

 will be thrown into violent vibration ; indeed it follows from (24) 

 that *j will be very small. The atoms will therefore behave 

 very nearly as if they were inelastic, and may remain long 

 enough in contact to assume a new condition of equilibrium by 

 uniting to form a single molecule. Exactly the reverse will 

 happen if m 1 is small compared with the mass of the inner 

 shells. 



We must therefore assume that in diatomic chemical molecules 

 the masses of the outer shells are very large compared to the 

 sums of the masses of the interior shells, while in the monatomic 

 molecules the masses of the outer shells are comparatively 

 small. 



We might now inquire why it is that in general more than 

 two atoms do not unite in this manner. To which the answer 

 is that the more complicated the structure of a molecule, the more 

 easily will it be broken up by the impacts of other molecules. 

 We must therefore assume that in the case of diatomic 

 molecules the violence and frequency of the impacts, even under 

 ordinary circumstances, are sufficient to break up any molecules 

 which may be formed containing more than two atoms ; while in 

 the case of other elements, such as arsenic and phosphorus, the 

 impacts are unable to break up the tetratomic molecules, even at 

 the high temperature of vaporization. 



In virtue of these considerations it appears that the formation 

 of a chemical compound, such as hydric chloride, is not such a 

 simple process as it was supposed to be in § 7. The impacts 

 will frequently produce diatomic molecules of hydrogen and of 

 chlorine respectively. The final condition of equilibrium will, 

 however, be arrived at on the same principle as before — namely, 

 that the molecules of hydric chloride are the least sensitive to 

 the action of light. Tetratomic molecules of hydric chloride, 

 will not be permanently formed, as the impacts, increased in 

 violence and frequency by the heat developed, will break them 

 1 up. Similar considerations apply to the formation of water. 



The formation of these simple compounds is, therefore, accom- 

 panied by, and due to the simultaneous breaking up of the 

 original diatomic molecules of the elements present. 



Double decompositions will take place in an exactly similar 

 manner, and considerations of the same kind apply to solid and 

 liquid bodies, in which, however, the impacts will be very much 

 less frequent. 



We also see that the broadening of the bands in the spectrum 

 of a gas, especially when due to a lowering of temperature, does 

 not necessarily show that the gas is a compound, as it may be 

 due to the union of previously dissociated similar atoms into 

 molecules. 



§ 10. Dissociative Action of Light and Heat. 



The fact that the same compounds which are formed by the 

 action of heat are again broken up when the temperature is 

 further increased, and, indeed, the dissociation of every chemical 

 compound at a sufficiently high temperature, is in apparent con- 

 tradiction to the conclusions of § 8. In the case of compounds 

 formed by the action of light it is quite possible that the internal 

 energy due to the action of heat may be greater than that of the 

 atoms at the same temperature. In general, it may be that 

 when the two constants c, (§ 1) combine to form one, the corre- 

 sponding critical vibrations are only produced at a much higher 

 temperature, and may then give rise to dissociation. Since, 

 however, all compounds are dissociated at sufficiently high 

 temperatures, there must be some other causes at work. We 

 may suppose that in gases at very high temperatures the mole- 

 cules are broken up simply by the violence of the impacts, and 

 this process would be facilitated by the molecules not being 

 spherical in form. 



The dissociative action of light observed in certain cases cannot 

 of course have a similar general explanation, and must not be 

 attributed to special chemical properties of light of certain wave- 



lengths, but to the values of the internal constants of the 

 molecules being of a kind specially favourable to such action. 

 Thus, as the author points out, we are led to the point of view 

 expressed by Lockyer, 1 as follows : — 



" The causes which are given in the text -books, showing us 

 the maxima of heat, light, and chemical action, are, I fancy, 

 merely causes showing us, as it were, the absorption spectra of 

 those substances by which the maxima have been determined — 

 whether they be lamp-black, the coating of the retina, or salts of 

 silver, and are really altogether independent of the nature of 

 light." 



§ II. Fluorescence. 



It has been pointed out in § 4 how critical vibrations may be 

 excited in a molecule by external disturbances, causing the mole- 

 cule to emit light of a certain wave-length. The disturbance was 

 supposed to be due to the action of heat, but from what has 

 gone before it is clear that they may be produced by ether 

 vibrations if only the molecule or atom is very sensitive to light 

 vibrations. For as soon as the impa'ct of light waves of a 

 certain (critical) vibration period has raised the internal energy 

 of the molecule to its maximum value, the molecule itself—that 

 is to say, its centre of gravity — will begin to execute vibrations ; 

 the different molecules will strike against one another, and the 

 result of these encounters will be to produce vibrations of the 

 other critical periods of the molecule, which will be different 

 from the vibration period of the impinging light. 



The substance will therefore emit rays different from those 

 which have fallen upon it. As a matter of fact some substances 

 having such special sensitiveness have been observed, 2 and are 

 known as fluorescent substances. The phenomena of fluorescence 

 must therefore be attributed to the absorption of light, as was 

 pointed out by Stokes. 



A fluorescent body is to be regarded as one in which the 

 molecular constants c z - have such values that the corresponding 

 light vibrations can be easily excited by external impulses. 

 Fluorescent substances must, in agreement with Stokes's con- 

 clusions, be regarded as being exceptionally sensitive. 



The theory does not lead to the law which has usually been 

 asserted, that the emitted light must necessarily be of longer 

 wave-length than the impinging light, and therefore the theory 

 is not inconsistent with Lommel's observations on naphthalan 

 red. 



Fluor-spar exhibits the phenomena of fluorescence to an ex- 

 ceptional degree. It may be that fluorine itself is exceptionally 

 sensitive to the action of light, and that the formation of the 

 mineral has not altogether destroyed this sensitiveness. If this 

 be so, it would explain the impossibility of preventing fluorine 

 from entering into combination with any substance with which it 

 is in contact. G. W. DE Tunzelmann. 



( To be continued. ) 



THE FORESTRY SCHOOL LN SPAIN. 



IN a Report to the Foreign Office which has just been published 

 the British Ambassador at Madrid states that Mr. Gosling, 

 First Secretary to the Embassy, has had the opportunity of 

 studying the excellent School of Forestry established at the 

 Escurial, and as great interest is now taken in forestal science in 

 England, and as efforts are being made to establish a British 

 National School of Forestry, he sends the information collected 

 by Mr. Gosling at an institution which, he thinks, is well 

 adapted as a type for a similar institution in England. 



Forestal legislation in Spain dates as far back as the close of 

 the fifteenth century -that is, in the reign of Ferdinand and 

 Isabella — and there is reason to believe that reckless destruction 

 of the rich forests was checked from time to time by Royal 

 ordinances. At the close of the sixteenth century Madrid was 

 surrounded by dense forests ; in fact, the city arms— a bear 

 climbing up a green tree — bear out the old chroniclers when 

 they speak of the forests which lay around the city, which 

 must have existed in the time of Charles V. So far is this from 

 being the case at present that for the most part the districts 

 around Madrid are treeless and unproductive, and as a conse- 

 quence exposed to the furious mountain storms, and unsheltered 

 in the scorching summer, whence comes the extreme unhealthi- 



1 "Studies in Spectrum Analysis," p. no. 



a Thomson mentioned, "Lectures on Molecular Dynamics, p. 280, that 

 his theory of absorption would account for the phenomena of fluorescence, 

 but he did not follow up the subject, j 



