104 
WNT TOL, 
[ DECEMBER 3, 1903 
electrification; cf. Amer. Jour. Sci., xv., 105) is rarely 
neutral as a whole. But it vanishes almost completely 
while the number of nuclei is relatively constant. In 
general, diminutions which are questions of seconds or 
minutes with the ions are more than questions of hours 
with the nuclei. 
Just as in these cases there is no marked decrease of 
the number of nuclei while the ions all but go, so I have 
been unable to find any contemporaneous increase of 
number; and yet in my experiments with phosphorus and 
with water nuclei the activities of any generator for the 
simultaneous production of nuclei and of ions seem to in- 
crease and decrease together. I shall be able to state this 
more definitely at the conclusion of my present experi- 
ments on the efficiency of different types of water jets. 
Finally, in my ‘‘ Experiments with Ionised Air’? (p. 12), 
I showed that in case of tests made with the steam jet, 
nuclei produced by the X-rays in atmospheric air were per- 
sistent in like degree with phosphorus and other nuclei. 
In fact, there was little difference in this respect among 
the nuclei examined. Nuclei produced in dust-free air, 
saturated either with water vapour or with hydrocarbon 
vapour, by the X-rays acting from without, retain the 
‘same order of persistence, whereas the ionisation is known 
to be fleeting. True, rubber stoppers and tubes made up 
a part of my condensation chamber, but in the case of 
water nuclei, at least, I can see no objection to this. The 
entire absence of electric field is always understood. 
In all cases, therefore, the electrification vanishes and 
leaves a nucleus behind, sometimes larger, sometimes 
smaller. If, in any one of them, the nucleation and the 
ionisation vanished at the same rate, the case would be 
good presumptive evidence of their identity. But, to my 
knowledge, never does this occur. What justification is 
there, then, to call the phosphorus nucleus an “ oxide,’’ or 
if an oxide associated with ionised air, why does one not | 
I should answer that the | 
phosphorus nucleus is the stable product of the initially | 
find the smaller air nuclei? 
jionised field. Again, why is phosphorus and dry air a 
more complicated system than air and water vapour under 
the action of the X-rays? Out of both systems eventually 
issues a stable nucleation. And why may one attribute to 
ionised air different condensational properties, according 
as positive or as negative ions are in question, without | 
having first established that the corresponding air nuclei 
do not differ in size sufficiently to account for the con- 
densational difference observed? Why may one condense 
on a nucleus from which the soul has fled and still be 
permitted to call it an ion? Why, indeed, does the nucleus 
persist after the ionisation has vanished; why does one 
not get back to dust-free air? My answer would be as in 
the case of phosphorus. As to water nuclei, I am much 
in doubt ever since I have been able to arrest the finest 
fog particles for examination whether the nucleus from 
shattered water is mere water dust. It seems to me, there- 
fore, that electrification, if present simultaneously with 
nucleation, is an incidental accompaniment with no 
immediate bearing on the condensation produced, and for 
this reason I have in the above endeavoured to account for 
the nucleus at the outset chemically. Cart Barus. 
Brown University, Providence, U.S.A. 
I po not think that any worker with ions or with con- 
densation nuclei who may have read the papers on ‘‘ Experi- 
ments with Ionised Air’’ and on ‘‘ The Structure of the 
Nucleus ’’ will consider my criticism unjust. 
The latter part of the letter requires some reply. Accord- 
ing to Prof. Barus, in all cases studied by him the nuclei 
were distinct from the ions, persisting long after the 
ionisation had disappeared. All that this proves is that 
he has not yet succeeded in observing condensation upon 
the ions, but only upon nuclei of another kind. According 
to my experiments (Phil. Trans., vol. cxciii. pp. 289-308, 
1899), a fourfold supersaturation is required to cause con- 
densation on the negative ions, a sixfold being required 
for the positive ions. To get such high supersaturations 
as these an exceedingly rapid expansion is required, and it is 
probable that the apparatus used by Prof. Barus is unsuit- 
able for the purpose. In the presence of any considerable 
number of nuclei requiring inappreciable supersaturation 
NO. 1779, VOL. 69] 
(as persistent nuclei always do) to cause water to condense 
upon them, it must be particularly difficult to reach the 
supersaturation necessary for condensation upon the ions. 
Such persistent nuclei always were present in Prof. 
Barus’s experiments; his failure to get condensation upon 
the ions was thus to be expected. His results have no bear- 
ing, therefore, upon the interpretation of my experiments 
on the action of the ions produced by X-rays and similar 
agents on condensation (for in these experiments nuclei 
more persistent than the ions were absent), nor of the ex- 
periments upon the charge carried by the ions made by 
Prof. J. J. Thomson (Phil. Mag., vol. xlvi. p. 528, 1898, and 
vol. v. p. 346, 1903) and by Dr. H. A. Wilson (Phil. Mag., 
vol. v. p. 429, 1903) with the same form of rapid-expansion 
apparatus as was used by me. 
I have never been able to produce by the action of 
X-rays nuclei other than the ions, but possibly very intense 
radiation may do so, as ultra-violet light certainly does. 
C. T. R. Witson. 
Cavendish Laboratory, November 23. 
Weather Changes and the Appearance of Scum on 
Ponds. 
Some experiments which I have been making during* 
the last year seem to bear very directly upon the 
interesting phenomenon described by ‘* Platanus orientalis ”’ 
in your issue of November 5. These experiments show that 
numerous solid substances suspended or dissolved in water 
have, by virtue of their surface-tension relations, a marked 
tendency to accumulate at any surface separating water 
from gas (vide Proc. Roy. Soc., August). Hence, by 
merely passing a stream of air-bubbles through solutions 
or suspensions of certain solids in water, it is possible to 
effect a considerable concentration of the dissolved or 
suspended solid in the upper layers of the liquid. Each 
bubble carries with it to the surface a load of solid particles, 
and leaves many of them floating there either as an ultra- 
microscopic ‘‘ pellicle ’’ or as a visible ‘‘ scum.”’ If a bubble 
is very minute, its load may be so great in relation to its 
volume that it may be entirely unable to rise, or may even 
sink. If, in these circumstances, the barometric pressure 
be diminished, the volume of the bubble increases in greater 
proportion than the surface-area, and therefore than the 
maximum load, with the result that numerous bubbles 
previously unable to ascend at once begin to rise towards 
the surface. If, during their ascent, the barometric 
pressure be sufficiently increased, at once they sink. If a 
vessel of water containing a sediment of sulphur or calcium 
soap, &c., be exposed to a sufficiently diminished air- 
pressure, the whole of the sediment will be seen to rise to 
the surface, the minute air-bubbles with their coating of 
solid acting like so many ‘‘ Cartesian Divers.’’ 
In every ordinary pond gas-bubbles of various kinds are 
constantly being formed by the action of micro-organisms ; 
in nearly every pond various solid substances, both organic 
and inorganic, possessing the required surface-tension re- 
lations, are present both in the mud and in suspension 
The gas liberated will be constantly bringing scum-form 
ing material to the surface, whether it rises in large masses 
or in small bubbles. Either a fall in the barometric 
pressure, or a rise in temperature, or an increase in the 
activitv of the gas-producing organisms should therefore 
result in increase of the scum. It must, however, frequently 
happen that the scum is swept to one side by the wind or 
sunk by various mechanical disturbances. 
It would be extremely interesting to learn whether by 
“decided change in the weather’’ your correspondent 
means a change attended by a falling barometer. 
Pembroke College, Oxford. W. RamMsDEN. 
The ‘‘ Affenspalte”’ in Human Brains. 
Witt you kindly allow me the privilege of using your 
columns for the following note? In a recent number of the 
Anatomischer Anseiger Prof. Elliott Smith published a most 
interesting forecast of an extensive work which he has in 
hand, dealing particularly with the occurrence in human 
brains of an occipital operculum; this occurrence had been 
considered previously as very exceptional, but Prof. Elliott 
Smith is able to show that this is far from being the case. 
