Marcu 25, 1897 | 
AI OM Cg 
485 
are dealt with. Of these there are upwards of 300, and 
a rough estimate of the total number of species in the 
world puts them at 3000. Prof. Hackel is a well-known 
and accepted authority on this difficult family, so that 
the translation will be welcome to those botanists who 
are not familiar with either Latin or German. The intro- 
ductory chapter on the structure, morphology and 
physiology of grasses enhances the value of this little 
book. 
It may be of interest to add, in this connection, that 
the grasses of British India, described in Sir Joseph 
Hooker’s voluminous “Flora,” just completed, number 
$50 species belonging to about 150 genera! 
W. B. H. 
The New Poultry Guide for British Farmers and 
Others. By Kinard B. Baghot-De la Bere. Pp. 65. 
(London : Seeley ana Co., Ltd., 1897.) 
‘THIS book is addressed to small landowners and tenant 
farmers ot Great Britain. It is a concise and practical 
guide to the selection and keeping of poultry for profit. 
Written by one who has had a wide experience, the book 
should appeal forcibly to the distressful agriculturist, 
and make him start a poultry farm at once 
LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice is taken of anonymous communications.) 
Liquefaction of Air by Self-intensive Refrigeration. 
From a recently published paper by Dr. Carl Linde, on the 
above subject, it appears that in his most successful attempt 
without the use of auxiliary refrigeration, he was able, with a 
-copper-tube apparatus weighing 132 Ibs., to liquefy air in two 
hours, with an average higher pressure of 190 atmospheres. 
It will be of interest to those who have followed this subject 
to hear of the latest performance, which constitutes a great 
advance on the above results; the weight of the copper coil 
having been reduced to less than one-sixth, the time required for 
liquefaction to less than one-fourth, and the pressure of the 
compressed air to less than one-half. 
An apparatus which I designed on an improved plan, and 
which was completed in May 1896, was exhibited at work at 
the conversazione of the Royal Dublin Society held on the 
1oth inst. ; and, the liquid being easily removable as produced, 
it supplied the material for repeated demonstrations during the 
evening, with the usual experiments. 
Air at an average pressure of 87 atmospheres was supplied by 
a compressor lent by Messrs. Arthur Guinness, Son, and Co., 
which had been formerly used for compressing carbonic acid gas ; 
and the compressed air was carried through 80 feet of copper 
pipe to the room in which the apparatus worked. No auxiliary 
cooling by carbonic acid or other agents was used to reduce the 
temperature of the compressed air before or after it reached the 
apparatus. 
The copper tube in the exchanger, weighing only 20lbs., was 
disposed in a special arrangement of coils, so that the tem- 
perature was exchanged over a range of 202° C. within 
14 of a degree, the compressed air entering at + 10° C., passing 
through the liquid state at — 192° C., and issuing a few seconds 
later at +8°6° C. 
When a start was made with the apparatus at atmospheric 
temperature, the jet of liquid air was clearly seen in twenty-five 
minutes, and the liquid was collecting in the receiver in thirty- 
three minutes from the start. When the apparatus was cooled 
down by continuous working, the liquid began to collect again in 
two minutes after emptying the receiver, and accumulated at a 
good rate : the exact quantities of liquid and air for a given time 
have yet to be measured. 
The receiver is a glass vessel protected by a vacuum of the 
kind invented by Mr. Crookes, first applied to refrigeration 
work by M. Cailletet, of Paris, and improved and popularised 
by Prof. Dewar. It is further protected by a special glass 
NO. 1430, VOL. 55] 
attachment fitted in such a way that the vacuum vessel can be 
readily removed without any risk of fracturing it by movement 
in contact with rubber hardened by cold to the rigidity of stone, 
and can be quickly replaced without interfering with the effective 
action of the apparatus. 
The apparatus has been very strongly and neatly made by Brin’s 
Oxygen Company, of Westminster, and tested to a very high 
pressure. It should be mentioned that in the almost impossible 
event of a joint giving way, no one can be hurt, since the high- 
pressure air exists only in the form of a thin column or thread 
with very small admission-passages. The harmlessness of a 
burst under these conditions has been practically demonstrated 
with a joint constructed in such a way as to burst inside a similar 
construction at 120 atmospheres, when the effect proved to be 
quite as mild as had been anticipated, and entirely harmless—a 
mere blowing off. 
This is the only apparatus existing in the United Kingdom 
which liquefies air without auxiliary refrigerating agents, 
March 13. W. Hampson. 
Patterns produced by Charged Conductors on 
Sensitive Plates. 
IN your issue of January 21, 1897, Mr. James I’Anson publishes 
some coin photographs showing the effect of the brush discharge 
around the edge of the coins, and around the portions in high 
relief, and asks if any similar results have been obtained by 
others. } 
In the Physical Review (vol. ii. p. 59, 1893) is an article on 
electric photography, in which are published some similar photo- 
graphs made by me in 1892 by exactly the same method de- 
scribed by Mr. ’Anson. Thesame method is also described by 
Prof. F. J. Smith, whose ‘‘Inductoscript” should be by this 
time well known in England. The rays from the discharge 
around the edge of the coin are plainly shown in one of my 
photographs in the Physecal Review, and are commented upon 
in the article. 
I also gave a photograph made by the same process when 
the coin was insulated from the photographic plate by a sheet 
of mica, and mentioned others made with the coin insulated from 
the plate by shellac, paraffin, and gutta-percha, which would 
seem to disprove Mr. I’Anson’s theory that the brushes are due 
to electrified streams of air coming in contact with the sensitised 
plate. 
: During the past year, I have repeated these experiments with 
both the coin and the photographic plate carefully insulated and 
placed between the plates of a condenser attached to the dis- 
charging knobs of a large induction coil. I have made in this 
way photographs of coins, and other conductors, imbedded in 
the centre of a block of paraffin two centimetres thick, under 
which circumstances they could not send off streams of electri- 
fied air. 
I have also repeated in this way some of the X-ray shadow 
effects by placing objects between the condenser plates and the 
photographic plate, to intercept the waves sent off from the con- 
denser plates themselves. A good conductor placed near the 
photographic plate will regularly be photographed more strongly 
than the condenser plates, even though it be only one thickness 
of gold-leaf on glass ; but if placed several centimetres from the 
photographic plate, and near one of the condenser plates, it may 
cast a shadow on the photographic plate. An insulator placed 
upon the photographic plate usually casts a shadow upon it, 
but in some cases insulators of high specific inductive capacity 
seemed more transparent than the air to the waves sent oft from 
the condenser plates. 
Since the oscillations in such a condenser field must corre- 
spond very closely in character with longitudinal waves in the 
ether, it seems probable that if X-rays are longitudinal ether 
waves, their wave-length must be very short ; as, otherwise, they 
would induce waves in conductors similar to those induced in 
an alternating condenser field. : ‘ 
I enclose a photograph of two coins placed side by side on a 
sheet of mica which was laid upon the photographic plate. The 
whole was placed in a light-tight box, and inserted between the 
condenser plates, from which it was carefully insulated by large 
panes of heavy plate-glass. The condenser plates were 4°5 cm. 
apart, and a 5cm. spark was passed between the discharging 
knobs of the coil for two minutes, after which the plate was taken 
out and developed in the usual manner. It was found later 
that an exposure of a few seconds gave equally good results 
