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ment that arose later he showed that his application 
for a patent was the result of work at Rothamsted 
with bones and mineral peers from 1839, and 
with bone dust from 1843. Liebig had suggested 
7“ manure in a report to the British Associa- 
in 1840, 
But in May 1837 an unnamed correspondent of 
the Farmers’ Magazine (2nd series), writing on the 
difficulty and expense of pearing bone dust in 
the required quantities, proposed the making of a 
“fictitious bone dust by impregnating lime with 
phosphoric acid.”” Another correspondent in answer 
asserted (May 1837) that there was no cheaper way 
_of getting phosphorus than by burning bones, adding, 
_ however, “ phosphate of lime if it céuld be found so 
as to be available to the farmer, would be invaluable. 
_ Whether it exists in England I know not, but in 
Spain there are entire mountains of it; it is com- 
pounded of phosphoric acid 41 parts, lime 59”’; 
showing an earlier appreciation in England of the 
fertilising possibilities of Spanish phosphorites than 
‘is enlly realised. 
ether or not Lawes had read these letters we do 
not know,,but they form an interesting foreshadow- 
_ ing of the great work he began two years later. 
, E. J. RussELv. 
A. HENDERSON SMITH. 
Rothamsted Expt. Station, 
Harpenden. 

Soil Reaction, Water Snails, and Liver Flukes. 
May I be allowed to add a few words to the dis- 
cussion on Limnea peregra and the liver rot of sheep, 
etc. First with regard to outbreaks of the disease 
_ following the application of lime. During a consider- 
_ able experience of Mid and North Wales I have had 
a number of such cases brought to my notice by 
_ farmers (in one case basic slag had been used). In 
all cases the dressings had been applied to rough wet 
astures of the ‘ sour”’ , which are not grazed 
sely by stock. In parts of these fields L. truncatula 
was et: but, owing to light grazing of the 
abundant herbage, the encysted cercarie had pre- 
Sumably not been ingested. Following an applica- 
tion of lime, a ‘‘ sweetening " or improvement of the 
t e leads to closer grazing and a more or less 
infection of the stock. This, at any rate, is 
my opinion following the investigation of actual cases. 
_ Secondly, as to the distribution of the two species 
_of Limnza (in the same regions). Both are abundant, 
_ and although they may occur together now and again, 
it is usual for L. peregra to frequent the softer muds 
and L. truncatula the firmer substrata. For example, 
if a small streamlet be followed, truncatula will often 
be found in its upper and peregra in its lower (and 
“more muddy) portions. In a wide ditch, truncatula 
may “ga the margins and peregra the soft central 
_ ~porti hese habitat differences are probably due 
to the relative size and expanse of foot. While working 
on the bionomics of truncatula I made some notes on 
gra also; these were incorporated in a paper 
shed in Parasitology, x., No. 2, December 1917. 
regard to peregra acting as an intermediate 
host Fasciola hepatica, 1 have on several occasions 
| obtained cercarie from that species which I cannot 
_ distinguish from that of F. hepatica (Cercaria fasciole 
_ hepatica, Thomas). This, however, is not a common 
_ occurrence in my experience, although I have ex- 
_ amined numerous samples of peregra. The last two 
_ eases were (a) from ill-drained fields, near Llanwnda, 
14 onshire, elevation about 100 feet ; (b) on the 
‘mountains near Bethesda, elevation more than 1000 
eet. In bothinstances liver rot had occurred. In the 
NO. 2778, VOL. 111] 
NATURE 
117 
first truncatula was present also ; in the second only 
peregra could be found. C. L. WALTON. 
Department of Agriculture, 
University College of North Wales, 
Bangor, January 15. 


The Silent Zone in Explosion Sound-Areas. 
In the recent interesting article on the Oldebroek 
explosion, it is stated (NATURE, January 6, p. 33) 
that in no case has it been found that the nearer 
margin of the outer sound-area lies at so short a 
distance as 114 km. from the source. When the 
minute-guns were fired at Spithead during Queen 
Victoria's funeral procession on February 1, Igor, 
there was a clearly marked silent zone, and the 
nearest point of the outer sound-area was 80 km. 
from the flagship. In this case the sounds were 
easily recognised, as they recurred at regular intervals 
(Knowledge, vol. 24, 1901, pp. 124-25; Science Pro- 
gress, Vol. 14, 1920, pp. 625-26), In the sound-area 
of one of the Asama-yama explosions (December 25, 
1910) the corresponding distance was about 87 km. 
(Bull. Imp. Earthq. Inves. Com., vol. 6, 1912, pp. 61- 
63 and plate 18). These figures have an important 
bearing on the origin of the silent zone. 
C. Davison 
70 Cavendish Avenue, Cambridge, 
January 12. 

Time Relations in a Dream. 
It is commonly believed that a dream which appears 
to be of long duration lasts in reality for a short time 
only. Since precise knowledge on the point is difficult 
to get, the following observations may be of interest. 
Having fallen asleep again, after being called a few 
mornings ago, I dreamt I was visiting a strange 
laboratory. On entering I was aware of a deafening 
hammering noise which rendered conversation im- 
possible. My host took me to another room, where 
the noise was inaudible, but on returning to the first 
room it continued, the blows being at about the same 
interval. I then noticed, what I had not seen before, 
some one striking a pipe in a shaft in the wall, but I 
reflected that the force used seemed quite insufficient 
to produce the sound heard. 
m awaking suddenly I connected the sound with 
the chipping of a stone-mason at work on the war 
jetcrtal across the road. Remembering Mr. J. 
Barcroft’s letter to NATURE (1919, vol. 104, p. 154), 
I timed the chipping blows. They were from 26-34 
per 10 seconds, averaging 3 per second. Going over 
the dream it seemed that the loud sounding blows, 
which produced a continuous reverberation, were 
about 15 or 16 per ro seconds ; thus the time in the 
dream proceeded at about—or possibly slightly less 
than—twice the normal rate. 
Both before and after the cessation in the dream 
—corresponding probably to one of the mason’s pauses 
—the rate was the same. In this respect the experi- 
ence differs from Dr. Barcroft’s, for his clock ticking 
four to the second appeared to give a five-second 
interval, namely a twentyfold exaggeration ; this, 
later in the dream, was reduced to a fourfold ex- 
aggeration. The noise of which I was conscious in 
the dream appeared to go on before the interval for 
about a minute and after it for two or three, with 
about a minute between. The duration of the dream 
appears accordingly to have been about two minutes 
or slightly longer. The loudness of the noise, as it 
was experienced in the dream, is remarkable in view 
of the actual loudness. The note was also far more 
metallic. W. R. G. ATKINs. 
Marine Biological Laboratory, Plymouth, 
January Io, 
D2 
