October 27, 1904] 



NA TURE 



641 



same, namely, the production of a complex organic com- 

 pound. We have been too much in the habit of supposing 

 that when we could represent the process followed by the 

 chemist by an equation, that this equation represented what 

 occurred in the plant or animal tissue. Physiologists more 

 than chemists have erred in this direction, and many of the 

 statements in our te.xt-booUs are either superficial or grossly 

 misleading. The chemist attains his end by violent means 

 and with considerable rapidity, whereas, in the silent 

 laboratory of the plant and animal cell, molecular processes 

 are slowly carried on of which we know at present next to 

 nothing. It is strange, for example, that we cannot yet 

 follow all the steps of the process by which, under the action 

 of sun-light, the green colouring matter in a vegetable 

 cell can fix the carbon of the carbonic acid of the air and 

 liberate the oxygen. Nor can we follow satisfactorily the 

 steps of the synthesis by which the carbon is built up into 

 such a substance as starch or sugar. Vet this is a synthesis 

 accomplished every day by every green plant. Such pheno- 

 mena in all probability are accomplished through the 

 agency of enzymes or ferments, but their real nature is still 

 obscure. 



I have said enough to show you the vast importance of 

 chemical investigation in the physiology of the future. 

 Chemistry is but a highly specialised branch of physics. 

 In these days all the new discoveries in physical chemistry, 

 such as the true nature of solution, the facts of dissociation 

 as exemplified by such a common phenomenon as the 

 splitting up of common salt into the ions chlorine and 

 sodium, the charging of each ion during electrolysis, 

 and the laws of osmotic pressure, which no doubt 

 regulate nutrition and the interchanges of blood and lymph, 

 must be taken into account by the physiologist. Such 

 research demands adequate laboratory accommodation and 

 highly trained specialists. I am glad to say our university 

 will soon be in a position to take her share in this 

 new development of physiological science. The splendid 

 laboratories now being built for physiology, public health, 

 and materia medica will be a home for work of this kind, 

 and the endowment of a lectureship in physiological 

 chemistry by the trustees of the late Dr. John Grieve (who 

 left 8000/. for the foundation of a lectureship in connection 

 with the medical faculty of the university) will enable us 

 to obtain the services of a trained specialist, who will give 

 his undivided attention to this department of physiology. 

 No subject more than physiology illustrates the truth that 

 all science is one. Physics, chemistry, physiology, and all 

 the others are only different ways of investigating the 

 phenomena of nature. The phenomena of life are, however, 

 the most difficult of all to investigate, and it may safely be 

 asserted that the highest skill in experimental research and 

 the deepest knowledge of chemistry and physics are re- 

 quired for such work. Throughout the scientific world 

 physicochemical researches are now in progress into 

 physiological and bacteriological processes, lectureships and 

 laboratories are springing up here and there, and it is 

 gratifying to be assured that the University of Glasgow 

 will be able to take her share in this work. 



CONDENSATION NUCLEI.' 

 A F.AMILIAR experiment was first shown illustrating the 

 "'"^ action of ordinary dust particles as condensation 

 nuclei. From a large globe, which had been allowed to 

 stand for some hours, some of the air was removed by open- 

 ing communication with an exhausted vessel. Only a very 

 few drops were formed as a result of the expansion. On 

 allowing air to enter the globe through a cotton-wool filter, 

 so that the pressure was brought back to its original value 

 (that of the atmosphere), and allowing the air to expand as 

 before, the drops formed were again very few. The ordinary 

 air of the room was now admitted ; an expansion of the 

 air in this case resulted in the production of a thick fog. 



When air has been freed from dust by filtering, or by 

 repeatedly forming a cloud by expansion, and allowing it 

 to settle, the vapour which, in the presence of the nuclei, 

 would have separated out in drops, must be in the " super- 

 saturated " condition immediately after the expansion is 

 completed. 



' Discourse Helivered at the Royal Institution on Friday, February ig, 

 by C. T. R. Wilson, F.R.S. 



NO. 1826, VOL. 70] 



.\nother method of producing clouds was now shown. 

 -■Vir was allowed to escape through a fine orifice into an 

 atmosphere of steam ; the mixed air and steam were then 

 passed through a Liebig's condenser, where the greater part 

 of the steam was condensed, and then into a large, glass 

 globe, where the clouds were observed. From this vessel 

 the air was drawn off by a pump which maintained the 

 pressure in the globe and condenser at a considerable number 

 of cms. of mercury below that of the atmosphere. Before 

 reaching the jet the air of the room had to pass through 

 a cotton-wool filter, and then through a long tube contain- 

 ing water ; finally it was led through an aluminium tube 

 to the orifice. The latter was about half a mm. wide. 

 I'he fall of pressure in passing through the orifice was about 

 15 or 20 cm. In the absence of the filter, the air being 

 admitted directly to the water tube through a tap turned 

 just sufficiently to give the same flow as with the filter, a 

 dense fog poured out from the end of the condenser tube; 

 on closing the tap and letting the air enter through the 

 filter the fog rapidly cleared, and only a fine rain continued 

 to be produced. While the apparatus was in this condition 

 an X-ray tube was set in action near the aluminium tube ; 

 the rain was succeeded by fog, which continued to pour out 

 from the end of the condenser so long as the X-rays were 

 kept in action. Condensation nuclei are, as this experiment 

 proves, produced in air exposed to Rontgen rays. Later 

 experiments will, however, show that they have entirely 

 different properties from the ordinary dust nuclei. 



When air has been completely freed from dust particles, 

 so that a slight expansion of the air (initially saturated with 

 water vapour) does not result in the formation of any drops, 

 it is found that quite a high degree of supersaturation may 

 be brought about without the appearance of a single drop. 

 There is, however, a limit to the supersaturation which can 

 exist without condensation of the vapour in drops resulting. 

 To study this condensation in dust-free air, and to measure 

 the expansion required to produce the necessary degree of 

 supersaturation, a special form of expansion apparatus is 

 required. The lantern slide thrown on the screen shows the 

 construction and mode of working of the apparatus. The 

 second slide is a photograph of the machine in action, the 

 exposure having been made immediately after an expansion ; 

 the cloud formed (in this case on nuclei produced by the 

 action of radium) is plainly visible along the path of a 

 concentrated beam of light from a lantern. 



Let us now try an actual experiment with the e.xpansion 

 apparatus. On making a slight expansion a cloud forms 

 on the dust particles which are present ; this slowly settles 

 to the bottom of the vessel. The air is allowed to contract 

 to its original volume, and a second expansion of the same 

 amount is made. The drops formed are on this occasion 

 comparatively few, and they fall rapidly ; the dust particles 

 have nearly all been carried down with the drops formed by 

 the previous expansion. The fewer the nuclei on which water 

 condenses the larger will be the share of water available 

 for each drop, and the more rapid will be the fall. The 

 next expansion produces no drops. While the air Is in the 

 expanded condition, the piston being at the bottom of the 

 expansion cylinder, air is removed from the cloud chamber 

 by opening the connection to the air-pump until the pressure 

 is about 13 or 14 cm. of mercury below that of the atmo- 

 sphere ; the piston Is again allowed to rise by putting the 

 air space below It in communication with the atmosphere. 

 The next expansion is thus comparatively large, the pressure 

 after the expansion has taken place and the temperature 

 has risen to its original value being 13 cm. or more below 

 the initial pressure. Yet, in spite of the high degree of 

 supersaturation reached, not a drop of water is seen. 

 Making the fall of pressure 16 cm., however, we see on 

 expansion a shower of drops ; and although these drops are 

 few and large, falling therefore rapidly, yet, however often 

 the same expansion be repeated, the drops produced on ex- 

 pansion show no diminution In number. Thus the nuclei 

 removed with the drops are continually replaced by others 

 manufactured within the apparatus itself. 



To produce the necessary supersaturation to cause con- 

 densation in the form of drops in dust-free air, the air must 

 be allowed to expand suddenly until the final volume is 1-25 

 times the initial volume. The condensation is rain-like in 

 form, and, moreover, the number of drops remains small 

 although the expansion considerably e.xceeds this lower limit. 



