August i6, 1900] 



NATURE 



367 



July 1896. In this Jpaper Mr. Fridlander gives the results of 

 his observations made during a voyage round the world from 

 this country to America, across that continent to Santa Cruz 

 Bay, from there across the Pacific Ocean to New Zealand, then 

 to Australia, and homewards by the Indian Ocean, Arabian 

 Sea and Mediterranean, visiting Switzerland on the way. On 

 the western side of the Atlantic the numbers were high, being 

 from 2000 to 4000 per c.c, though the vessel was far from land, 

 its position being 55° o' N., 42° 11' W. Lower numbers were 

 obtained between Labrador and Newfoundland, the readings 

 there being from 420 to 840 per c.c. Readings as low as 280 

 per C.C. were got in the Gulf of St. Lawrence. On the Pacific 

 coast the lowest was 700 and highest 4500 per c.c. On the 

 Pacific Ocean the lowest reading was 280 per c.c. and highest 

 2125. Few readings were obtained in New Zealand under 

 1000 per c.c. In the Indian Ocean the air seems to be rather 

 purer than most places, or at least was so when the observations 

 were made. Readings as low as 200 per c.c. were obtained, 

 arid they seldom were over 500. Tests made on only two days 

 in the Arabian Sea gave a minimum of 280 and a maximum of 

 1375. One day's tests in the Red Sea gave from 383 to 490 

 per c.c. The result of two days' tests of the Mediterranean 

 air gave a minimum of 875 and a maximum of 2500 per c.c. A 

 result which agrees with that already given for the French 

 coast of the Mediterranean. 



Mr. Fridlander's tests of the air in Switzerland give results 

 similar to those already referred to for the Rigi Kulm. At 

 almost all the places the numbers were always over 1000 per c.c, 

 though the observations were made at considerable elevations. 

 But on the Riffelberg (altitude 7400), where Mr. Fridlander 

 spent some days, the numbers varied from 225 to 4000 per c.c. 

 On the summit of the Bieshorn (altitude 13,600) the lowest 

 observation gave 140 per c.c, which, so far as I know, is the 

 lowest number yet observed in Switzerland. When we com- 

 pare the figures given by Prof. Melander and Mr. Fridlander of 

 the dust particles in the air of different parts of the world with 

 those obtained in the Atlantic air on the west coast of Scotland, 

 we are forced to admit that the latter is abnormally pure. 



The rate of fall of cloud particles as given by the calculations 

 of Mr. Wilson seems to be much too rapid. He assumes that 

 the air in which clouds are formed is always rising. This can 

 hardly be said to be the fact. Suppose a large area of the 

 earth's surface to be covered with cloud, forming a vast sea, 

 such as one sometimes sees from the top of a mountain. It is 

 evident that the air over all that area cannot be rising at any 

 considerable rate, and yet the clouds will be seen to keep nearly 

 the same elevation for hours. If the air be still, and if Mr. 

 Wilson's calculations are correct, then the mountains ought to rise 

 out of such a cloudy sea at the rate of nearly 500 feet per hour, 

 a phenomenon which, I venture to say, no one has ever seen. 



Mr. Wilson seems to think, though all the dust particles in 

 cloudy air will not become centres of condensation, it is a matter 

 of no importance, as he thinks the cloud will act as a perfect 

 filter, by the descending cloud particles coming in contact with, 

 and absorbing, the inactive dust particles. So that all particles 

 that do not become active centres of condensation will be carried 

 out of the air by the falling drops, and leave the air rising 

 through the cloud particles dustless. He gives no evidence in 

 support of this assumption other than the purification of dusty 

 air in a closed vessel with wet sides. Now dusty air in a closed 

 vessel takes a considerable time to become dust-free, and I 

 think it may be contended that gravitation plays no inconsider- 

 able part in the process, perhaps more than the wet sides 

 referred to by Mr. Wiison. So far as my observations go, there 

 is no evidence of any such powerful purifying effect in clouds. 

 At least when making observations in old clouds, both at top 

 and bottom of them, there were always observed a large number 

 of dust particles, but whether any had been absorbed by the 

 cloud particles or not it would be impossible to say. If any had 

 been absorbed, certainly many were still free. 



That clouds have not the purifying effect claimed for them by 

 Mr. Wilson may be best shown by reference to the observations 

 made on the Rigi Kulm on May 21, 1889 {Proc. Roy. Soc. Edin., 

 vol. xvii. p. 193). On the morning of that day, when I left 

 Lucerne on my way to the Rigi Kulm, the sky was covered 

 with cloud, and when ascending the mountain the cloud was 

 entered at an elevation of about 2000 feet below the top. On 

 arriving at the top the clouds were still very dense, and remained 

 so till the evening ; afterwards they settled down to the level of 

 the kulm, when a vast sea of clouds was disclosed stretching in 



NO. 1607, VOL. 62] 



all directions with the peaks of the higher Alps standing out like 

 islands. Under these conditions the observations made on the 

 top of the Rigi on that day were evidently taken in air just 

 above the upper surface of a uniform stratum of cloud 2cxx) feet 

 deep, where, according to Mr. Wilson, there ought to have been 

 dustless air, yet the observations showed there were still 210 

 particles per c.c. Next morning the clouds still extended in 

 most directions and were much thinner, and the number of dust 

 particles had increased to over 800. 



I may as a well here call attention to the fact that during the 

 night the upper surface of the clouds had only settled down about 

 icxx) feet. How much of this was due to the cloud particles 

 falling through the air, and how much to evaporation, it would 

 be hard to say. Probably evaporation played the principal part, 

 as the clouds were now much thinner, and the evaporation 

 probably took place from the upper surface, as in the morning 

 the air on the Kulm was dry — the wet-bulb depression being as 

 much as 6°, and a wind of some strength was blowing from the 

 south-east. The rate of descent of the particles in this cloud 

 was therefore much slower than the rate of fall calculated by Mr. 

 Wilson. 



Mr. Wilson, in criticising my remarks on the re-evaporation 

 of cloud particles, says : " But all drops that have survived the 

 great tendency to evaporate which accompanies the initial stages 

 of their growth will surely continue to grow so long as the rate 

 of expansion remains the same, or even if it be much reduced." 

 Here again Mr. Wilson assumes that clouds are always rising. 

 Now a great part of the life of, a cloud, and the air in which 

 the particles are carried, is spent in moving horizontally, and 

 sometimes even downwards, and occasionally with but little 

 movement in any direction ; and it is during this stable con- 

 dition that the opportunity is given for the re-evaporation of the 

 smaller drops. Mr. Wilson points out that if a very slight 

 proportion of the water in a drop were to evaporate, it would 

 cool the drop and check the evaporation, a statement with 

 which all will agree. But though the cooling may check 

 evaporation, it will not stop it. The particles in a cloud are 

 close together, and those condensing vapour and .growing 

 warmer soon part with their heat by radiation and by contact 

 with the air, so that the heat lost by the evaporating particles 

 is rapidly supplied to them by the condensing ones, and, as 

 we shall see later, this exchange of heat takes place at a much 

 quicker rate than one might imagine. 



I do not think that practical chemists will agree with Mr. 

 Wilson's statement that all the ammonia, nitric acid and other 

 impurities, out of which the sun can manufacture nuclei, will be 

 washed out of the air by the rain. The difficulty of removing 

 the last traces of gases by washing is well known. 



Are meteorologists prepared to accept that part of Mr. 

 Wilson's theory which necessitates the formation of rain-clouds 

 at an elevation of 7500 feet above the topof the ordinary cumulus 

 and nimbus clouds ? In other words, are meteorologists prepared 

 to affirm that there are two distinct rain zones— one where the 

 ordinary rain-clouds condensed on dust nuclei are formed, then 

 over these clouds clear air for 7500 feet, above which the ion 

 rain-clouds are formed ? This upper ion-cloud must result in 

 rain if the theory is correct, otherwise there will be no separation 

 of the positive and negative ions. I lea%'e it to the meteorologists 

 to say whether rain-clouds have ever been observed at elevations 

 of 20,000 to 30,000 feet — not above sea-level, but above the 

 surface of the ground. 



Mr. Wilson does not seem to think that my remarks on the 

 rapid growth of cloud particles in supersaturated air have any 

 bearing on the subject, and objects to my use of the term ex- 

 plosive in reference to the condition of supersaturated air. If 

 I had known a better term I would have used it. Though super- 

 saturated air is in a condition of equilibrium with itself, yet when 

 nuclei are introduced into it there is at once a rapid rush of 

 vapour molecules towards the condensing particles, and a rapid 

 breakdown of conditions all round the nuclei, which seems to 

 me not at all inaptly compared to an explosion— centripetally, 

 of course. Mr. Wilson grounds his objection to the rapid 

 growth of the ion-cloud particles in supersaturated air on the 

 difficulty and slowness with which the condensing drops part 

 with the heat developed by the condensing vapour. I shall not 

 follow Mr. Wilson in his comparison of a condensing with an 

 evaporating drop, as it is not easy to see the changes taking 

 place in the latter, but will rather refer to an experiment which 

 Mr. Wilson, and others who have experimented on this subject, 

 must often have seen. Take a glass flask in which there is a 



