NovEMBER 12, 1896] 
NATURE 
22 
pore) 
A striking contrast is afforded by placing a piece of plain 
white paper over a portion of the screen, and observing the 
whole by the light either of a Geissler tube or of the Tesla 
brush. 
Under these conditions the white paper will appear very dark, 
while the fluorescent surface is brilliantly luminous. | 
A. A. C. SWINTON. 
66 Victoria Street, London, S.W., November 3. 
IN continuation of ‘our letter of October 29, we find that a re- 
flexion grating does not show the bands so well asa quartz prism, 
because metals, ¢.g. silver-on-glass, do not reflect all this kind of 
light completely except at grazing incidence. The furthest band 
shown by a Rowland grating at incidence 45° has wave-length 
2200 tenth-metres, whereas the extreme ultra-violet usually 
quoted (the Fraunhofer line U) is 2948 tenth-metres, and the 
bright bands shown by quinine are about 3250 and 3830 
respectively. With a quartz prism the fluorescence caused by 
light of shorter wave-length than 2200 can beseen. But it is 
quite possible that Sir George Stokes in 1852 saw as far as we 
can see to-day. OLIVER J. LODGE. 
November 7. BENJAMIN DAVIES. 
Osmotic Pressure. 
As Mr. Whetham has called attention in your columns to 
my attempt to give a mechanical hypothesis for osmotic pressure, 
in the October number of the Phzlosophical Magazine, perhaps 
you will allow me to give some explanation of my somewhat 
faulty use of the term ‘‘ dissociation hypothesis,” which, as Prof. 
Ramsay has pointed out to me, may easily be misunderstood. 
I have used the term to signify not the separation of the ions 
in electrolytes, but rather the freedom of the solute molecules 
in non-electrolytes, and of the atoms in electrolytes—their 
dissociation, in fact, from the molecules of solvent. It appears 
to me that in some statements of the facts of osmotic pressure, 
the idea is strongly suggested that the molecules or atoms of 
the solute are moving about among the molecules of the solvent, 
and, as far as pressure at any rate is concerned, dissociated from 
them, and producing an independent effect, the osmotic pressure 
being directly due to the solute. My aim is to show that we 
may more reasonably account for the facts by supposing that 
the solute molecules or atoms are associated with the solvent 
molecules, entering into some kind of more or less unstable 
combination with them, and that the solution is not to be 
regarded as consisting of two parts producing independent 
pressures. The extra osmotic pressure is, of course, due to the 
solute in one sense, in that it would not exist without it ; but it 
is an indirect effect, due to the modified compound molecules 
formed. The first effect is a decrease in ‘‘ mobility” of the 
solution, so that the exchange in the two directions through a 
semi-permeable membrane is unequal, and it is only rendered 
equal when the solution is put under the extra pressure which 
we call osmotic pressure. 
Mr. Whetham has shown, in a very simple way, that my 
hypothesis does not necessarily conflict with the facts of electro- 
lysis, and that the idea of dissociation of the ions from each other 
may easily be reconciled with it. J. H. Poyntinc. 
Mason College, Birmingham, October 31. 
“Purple Patches.” 
I sHOULD be very glad if I could obtain information as to the 
cause and nature of certain ‘‘ purple patches” which I have 
noticed from time to time for many years past, but have been 
unable to get explained. The patches in question occur during, 
or immediately after, rain, on the pavement or roadway ; dashes 
of vivid purple, or rather violet, varying in size from small splashes 
or drops to patches as large as the palm of one’s hand, but most 
commonly they are about the size of a shilling. When quite fresh, 
sometimes a little clot is observable in the centre of the splash. 
Sometimes I find one patch completely isolated, sometimes 
two or three in close proximity ; sometimes, again, numerous 
little drops scattered over a certain space; once I counted 
twenty or thirty tiny dashes in about ten yards of pavement. 
When quite wet the violet colour can be rubbed up with a hand- 
kerchief or paper, which it stains as with “‘aniline purple” dye, 
as it does the pavement, and when once dry it is quite inerasible, 
and lasts till it is worn away by exposure, or the feet of passers- 
by. I observe it to occur chiefly during warm rain after a dry 
or cold spell; xever during dry weather, whether in summer or 
winter. During the past hot summer there was none to be 
NO. IAII, VOL. 55] 
’ 
found, but directly the weather changed in July, I saw it in 
various localities. This was also the case in the long cold winter 
of 1895, when on the breaking up of the frost there were 
plentiful patches to be seen up and down the streets ; there was 
also a complete absence during the following summer, till the 
drought gave, and then again I found this appearance recur. I 
naturally observe it most in Bath, where I live ; but it is not at 
all confined to one place or situation. I have found good 
specimens at such widely different places as the doorway of a 
hotel at Oban ; the Castle Hill, Edinburgh ; railway platform at 
Morecambe ; doorstep at Windermere ; in streets and roads at 
Cambridge, Bude, Penzance, St. Ives, Clevedon; once in a 
London street (Pall Mall East), and once some was found in a 
cold water bath. 
I have from time to time made inquiries from various people 
who I thought would know, but have not been fortunate enough 
to meet any scientific person who has observed it. But one 
learned professor to whom I described the ‘‘ patches,” suggested 
whether ‘‘ purple bacteria’? would prove a solution to the 
mystery, and recommended me to inquire through the medium 
of your columns. I should be much obliged if some one would 
enlighten me, or mention some authority to whom I could 
refer. A. PEDDER. 
13 Somerset Place, Bath, October 27. 
Note on ‘“‘ Plasmodiophora brassice.” 
ALTHOUGH it is well known that Plasmodiophora brassice 
attacks the great majority of cruciferous plants, yet no instance 
of the common Shepherd’s purse (Capsed/a bursa-pastoris, D.C.) 
being attacked is recorded in this country. Thus Massee (Prec. 
Roy. Soc., vol. lvii.) quotes the Shepherd’s purse as being reported 
by Halsted to be attacked in America, but says ‘‘ It has not 
been observed to be diseased in this country, although one of 
our commonest weeds.” During the past summer my attention 
was drawn to some plants of Cafse//a with swollen roots, grow- 
ing in a sandy field near Coventry, on land upon which crops 
of swedes and turnips were grown in the usual rotation, and I 
had no difficulty in finding several additional specimens. These 
roots, On examination, were found to have the characteristic 
plasmodium in their cells. The Shepherd’s purse must now be 
numbered among the plants in this country which provide a 
home for Plasmodiophora, and probably help it to maintain its 
existence in the ground from year to year, thus proving a 
possible source of injury to cruciferous crops. 
The length of time for which Plasmodzophora can retain its 
vitality in the soil in the absence of any cruciferous plants, is 
still a matter of uncertainty. In order to ascertain this, in 
November 1893, I established a series of experiments (fully de- 
scribed in my annual report to the Newcastle Farmers’ Club, 
1895), intended to extend over a period of six years. In these 
experiments, six beds (A-F) and six large 18-inch flower-pots 
(A-F) were prepared, and each strongly infected with pieces of 
turnip badly diseased with ‘‘ Finger-and-Toe,” one bed and one 
flower-pot being sown with turnips each successive spring. The 
beds and pots acted as duplicate experiments, and the soil in 
both remained unmanured, and was carefully guarded from the 
intrusion of P¥lasmodiophora, while all cruciferous plants were 
rigidly excluded. 
In 1894, in both pot A and bed A ‘‘ Finger-and-Toe” ap- 
peared ; in 1895, in the pot B ‘‘ Finger-and-Toe” was found 
upon four plants out of six; and in the bed B, 8 per cent. were 
diseased. In 1896 the same result was strikingly shown ; after 
a period of three years, the bed C and pot C were still found to 
be diseased, four out of five plants being affected in the latter, 
and 10 per cent. in the former. Massee had previously shown 
that the germs of disease retain their vitality for two years, and 
my experiment this year shows that this period can be increased 
to at least three years. M. C. Porrer. 
Durham College of Science, Newcastle-upon-Tyne, October 24. 
Sparrows and Wheat. 
In vol. vii. part iii. p. 522 of the /owrnal of the Royal 
Agricultural Society it is stated that, in the Leicester district, 
Rivett’s wheat is much grown, the reason being that sparrows 
do not attack it, while they do other varieties. Can any of your 
readers assign a cause for the exemption from attack of this 
particular variety ? It is most curious if correct; and the 
authority quoted apparently is a good one. 
ia r F. G. BRook-Fox. 
Port Navis Cove, Penryn, Cornwall. November 1. 
