March 22, 1906] 



NA TURE 



499 



the luminosity begins there is such a rapid increase in 

 the ionisation that the current through the gas and the 

 rate of doing work increase in an exceedingly short time 

 through a wide range of values, and thus a gradual in- 

 crease in the rate of work is exceedingly difficult to 

 obtain. On several occasions, however, I was convinced 

 that on gradually increasing the rate of work the mercury 

 lines were the first to appear, and were the last to dis- 

 appear when the rate of work was reduced from a high 

 value, at which both the nitrogen and mercury spectra 

 were bright, down to a point where the discharge ceased to 

 be luminous. 



The preceding considerations have also an important 

 application to the difference between the arc and spark 

 spectra. In the continuous arc discharge, although the 

 average rate of work is much higher than in the spark, 

 the maximum rate is very much less ; in the spark dis- 

 charge we have an exceedingly intense current density last- 

 ing for a very short time, and while the spark is passing we 

 have a very much greater rate of work than in the arc. 

 Hence the state of things in the spark will be analogous 

 to that represented in Fig. 5, and the lines corresponding 

 to systems of the type B will be enhanced relatively to 

 those of type A ; we conclude, then, that the arc lines 

 correspond to systems of the type A, the spark lines to 

 those of type B. 



The work done in the discharge tube is probably ulti- 

 mately converted for the most part into heat, so that the 

 rate at which work is being done at any part of the tube 

 is approximately proportional to the rate at which heat 

 is being produced in the tube. I do not, however, regard 

 temperature, i.e. the energy due to the translation of the 

 atoms as a whole, as having any direct connection with 

 the production of spectra. The work done by the electric 

 field on the corpuscles is, since the corpuscles can easily 

 penetrate the atoms of the gas, first converted into internal 

 atomic energy ; this energy may ultimately be for the 

 most part transformed into the energy of translation of 

 the molecules of the gas, and so appear as temperature, 

 but it by no means follows that if we heat the molecules 

 of the gas by non-electrical means to the temperature to 

 which even a few of its molecules are raised by the 

 electric discharge we shall get a luminous spectrum. The 

 production of the spectrum depends upon the internal 

 energy of the atom ; when we use the electric discharge 

 all the work done by the corpuscles goes at first into the 

 form of internal atomic energy, while if we supply the 

 same amount of energy to the gas by thermal, as dis- 

 tinguished from electrical, means, the energy will go 

 first into increasing the energy of translation of the atom, 

 and very little of it will ever get inside the atom. It 

 is probable, however, that some of the energy of trans- 

 lation will get converted into internal energy, and that 

 temperature is one way of giving internal energy to the 

 atom, and so producing luminosity ; from our point of 

 view, however, it is a very extravagant method, as the 

 fraction ol the energy spent in heating the gas which 

 goes to produce luminosity is small. 



The coefficient of absorption a of the systems will de- 

 pend upon the way in which the internal energy is given 

 to the atom as well as upon the rate at which the electric 

 field is doing work in the neighbourhood of the atom. 

 Thus, for example, if the internal work is given by means 

 of rapidly moving corpuscles, the coefficient of absorption 

 will depend upon the velocity of the corpuscle, for we 

 can easily show that when a corpuscle passes at a fixed 

 distance from a system of corpuscles having a definite 

 period of vibration there is one velocity of the corpuscle, 

 depending on this period, fast if the period is short, slow 

 if it is long, for which the energy given by the corpuscle 

 to the system is a maximum. Thus the relation between 

 the amounts of energy absorbed by two systems from the 

 corpuscles depends upon the velocity of the corpuscles. 

 The velocity of the corpuscles in a discharge tube depends 

 upon the pressure of the gas, so that even though the 

 rate at which the electrical forces are doing work may 

 be the same at two different pressures, the relative intensi- 

 ties of the lines of two systems A and B may be different. 



Again, we might expect that the coefficient of the rate 

 of absorption of energy would be different according as 

 the energy is given to the atom by means of the large 



systems which form the positive ions or by means of 

 small corpuscles, and that the relative brightness of lines 

 might be different in the two cases. In the Kanal-strahlen 

 we have positive ions moving through a gas and producing 

 luminosity, and the spectrum of this luminosity possesses 

 interesting peculiarities differentiating it from the spectrum 

 of other parts of the tube. Perhaps the most striking 

 difference, however, is when the positive ions strike against 

 a salt like lithium chloride; they make the red lithium 

 line appear with great brilliancy, while if corpuscles strike 

 against the chloride the red line is not visible. It is re- 

 markable that the spectrum of the metal is produced much 

 more readily by the positive ions when they strike against 

 a salt of the metal than when they strike against the 

 metal itself ; this is shown in a striking way if we take 

 the liquid alloy of sodium and potassium and direct a 

 stream of Kanal-strahlen upon it ; the clean parts of the 

 alloy appear quite dark, but the specks of oxide scattered 

 over its surface shine with a bright yellow light, giving 

 the sodium spectrum. 



When the internal energy of the atom is increased by 

 means of light, as in Prof. Wood's beautiful experiments 

 on the fluorescence of sodium vapour, the coefficient of 

 absorption of a system will depend upon the relations 

 between the periods of that system and the period of the 

 incident light vibrations ; thus, as Prof. Wood found to be 

 the case, the numerous lines in the spectrum given out by 

 the vapour alter greatly in character and wave-length when 

 the period of the incident light is changed. 



The same principles which explain the variation in the 

 intensities of the spectra given out by two different systems 

 in the same atom can be applied to explain the variations 

 in the intensities of the spectra of two gases, A and 

 B, when these are mixed together. We know that under 

 some conditions the lines of only one constituent of the 

 mixture appear, while under others we get the lines of 

 both the gases. Let us suppose that the lines of A appear 

 with a lower rate of work of the electric forces than 

 those of B, and that we send a constant current through 

 the discharge tube, we can calculate what the electric force 

 must be to produce from the molecules of A alone the 

 number of ions required to carry this current ; having 

 founa the electric force on this supposition, we can, know- 

 ing the current, find the rate at which the electric forces 

 woula be doing work in the tube ; if this rate of work 

 is less than that required to make B luminous, the current 

 will be carried by the ions of A alone, and the spectrum 

 of B will not be developed ; if the rate of work on this 

 supposition is greater than that required to make B 

 luminous, the spectrum of B will appear, and it must 

 take a share in carrying the current. Let us suppose that 

 we have so much of A present that the rate of work is 

 not sufficient to develop the spectrum of B, and consider 

 what will happen as the proportion of A is diminished. 

 In order to supply the number of ions required to carrv 

 the given current from the smaller number of molecules 

 of A, the electric force, and therefore the rate of work 

 in the tube, must, on the supposition that the current 

 is wholly carried by A, increase, and if we con- 

 tinually diminish the amount of A present the rate of 

 work will at last reach a value sufficient to make B 

 luminous with the given current. This stage will give 

 the smallest quantity of A which can for the given current 

 wholly swamp the spectrum of B. The rate of work done 

 in the tube will depend on the current going through it 

 and also on the pressure of the gases, so that both these 

 quantities will influence the proportion of the gas B re- 

 quired to make its spectrum visible. 



MICROSCOPIC AOUATIC PLANTS AND THEIR 



PLACE IN NATURE. 1 

 T7VERY piece of water, besides containing large plants 

 and animals which are readily visible to the naked 

 eye, harbours a more or less considerable number of minute 

 forms, which pervade all the layers of the water in vary- 

 ing amount, and collectively constitute the plankton or 

 pelagic life. The most important difference between the 

 1 Abstract of a lecture on "The Microscopic Plants of our Waters," 

 delivered befoie the London Institution on February 1 by Dr. F. E. 



NO. 1899, V0L 73] 



