224 
The men allege that this ‘‘ foul water’? has an injurious effect 
POAT ORE 
upon their tackle, and also lessens their take of some kinds of | 
fish. It continues for about a month and then disappears. This 
year I have had ard still have some of the organisms under 
microscopic observation, and I am very anxious to know if they 
have already been the subject of scientific inquiry, or not ; and 
also information as to the geographical extent of their diffusion. 
-> Sheerness-on-Sea W. H. SHRUBSOLE 
Composite Portraits 
If is most unfortunate, but an obvious fact, that in the sheet 
of composite portraits of American notabilities in NATURE of 
June 25, Figs. 2 and 3 are impressions from one and the same 
negative. Not only are they alike, but they present the same 
peculiarities, even the same defects. If it were not so, they 
would serve to blow to shivers the whole edifice founded upon 
such averages ; for if 16 naturalists and 31 academicians present 
two composites which are indistinguishable, to what purpose is 
the average ? C. M. INGLEBY 
Athenzeum Club 
IRIDESCENT CRYSTALS OF CHLORATE OF 
POTASH 
“Tee appearance of Mr. Madan’s paper in NATURE, 
vol. xxxli. p. 102, induces me to offer some addi- 
tional remarks on this subject. 
[Fuly 9, 1885 
should furnish the key to the explanation of the colours 
of the chlorate crystals. 
In truth, I do not see how the presence of tubes, if such 
there be in the opal, would account for the phenomena, 
| and especially for the very peculiar spectrum exhibited. 
The supposition of the existence of rows of tubes leads 
one to look in the direction of diffraction. But I do not 
see how monochromatic light, or, at least, light almost 
monochromatic, can be obtained by diffraction. And 
even independently of this consideration there is one 
feature of the production of colour in the chlorate crys- 
tals which shows, at once and decisively, that at least in 
their case the colour cannot be due to diffraction. If an 
iridescent crystal be chosen with an even surface, and the 
flame of a candle in a dark room be viewed by reflection 
in it, it is found that the colour is seen in the direction of 
the regularly-reflected light. In fact, the coloured light 
forms a well-defined image of the flame of the candle, 
coinciding with, or overlapping, the colourless image due 
| to reflection from the first surface. This differs altogether 
| from what we get in the case of a grating, or in that of 
In the discussion that followed the reading of my paper | 
Mr. Crookes referred to the closely analogous spectra 
exhibited by opals, as described in his paper (Prac. Roy. 
Soc., vol. xvii.). This paper, though it came before me 
at the time when it was read, was not in my mind when I 
wrote my own. I called shortly afterwards at Mr. 
Crookes’ house, and saw the spectra of his opals. 
posing that there were sufficient grounds for the commonly 
received idea that the colours of the opal are due to fine 
tubes in the mineral, we did not at the time conceive 
that the phenomena could be the same; were it not for 
Sup- | 
this, I should certainly have added to my paper a refer- | 
| from saying anything about it in my former paper, nor 
Mr. Crookes was so good as to lend me his opals for 
ence to that of Mr. Crookes. 
more leisurely study. 
impressed me with the similarity of character of the 
The further examination has so | 
spectra, that I am strongly disposed to think that the | 
colours of the opal and those of the chlorate crystals may 
be due to the same cause. This does not, however, lead 
me to attribute tubes or striz to the chlorate crystals, | 
the structure of which can comparatively easily be made 
out, but to doubt very greatly the theory which attributes 
the colours of opal to fine tubes. 
Mr. Madan does not profess to have actually seen in 
the chlorate crystals such tubes as he supposes to exist, 
nor could I see anything of the kind on examining some 
of the crystals I have got after the appearance of his 
paper. On the other hand, I notice that Brewster did 
not state that he had actually seen the supposed tubes, 
but merely inferred their existence from a comparison of 
the appearance under the microscope of the precious opal 
with that of hydrophane. And Mr. Crookes tells me that 
an opal is not spoiled or affected by being immersed in 
water or even oil. The fact is that it is extremely difficult 
to make out what the actual structure is with which we 
have to deal in the case of the opal, whereas in the case 
of the chlorate crystals it is unmistakable. Moreover, in 
the case of the chlorate crystals there is a wonderful uni- 
mother-of-pearl or Labrador spar. It agrees so far with 
the colours of thin plates, or the colours shown by reflec- 
tion by certain quasi-metallic substances, such as several 
of the aniline dyes, though the production of colour in 
these three cases is due to three totally different causes. 
It has been conclusively proved that the seat of the 
colour in the chlorate of potash crystals is in a very thin 
twin stratum ; and I entertain myself little or no doubt that 
the colour depends in some way on the different orienta- 
tion of the planes of polarisation in the two components 
of a twin, and on the difference of retardation of the two 
polarised pencils which traverse the thin stratum. But 
anything beyond this is at present only a matter of specu- 
lation. I see only two directions in one or other ot 
which to look for a possible explanation; but as these 
could only be propounded at considerable length, and the 
matter has not at present advanced further, I refrained 
will I further mention it here. 
In conclusion, I would mention an interesting paper on 
“ The Spectrum of the Noble Opal,” by Prof. H. Behrens, 
a copy of which I have just received by the kindness of 
the author. In this paper, which is printed in the /Veves 
Fahrbuch fiir Mineralogie, &c., 1873, the author, who 
was evidently unacquainted with Mr. Crookes’s paper 
when he wrote his own, has described and figured the 
peculiar spectra of several opals. G. G. STOKES 
EXPERIMENTAL FARMING 
NGLISH farmers are not readers. They do not 
know, apparently, that there is much to unlearn in 
the practice of their art, old as it may be. But although 
they will not allow themselves to be enlightened by books 
or newspapers, they are not incapable of imitation, and 
for that reason experimental farming, carried out in any 
particular district in a practical manner, has always proved 
useful. Up to the present time the history of experimental 
farms in England is, so far as their number is concerned, 
a meagre one. In the whole of Europe there are 160 
experimental farm stations, of which number the United 
| Kingdom can boast of about half a dozen, including 
formity in the phenomena presented by the same crystal, | 
extending, it may be, over nearly the whole of even a 
large crystal, whereas in the opal the colour extends over 
comparatively small patches ; and even a single patch is 
seen under the microscope to present differences of struc- | 
ture in different parts. Hence if the colours in opal and 
those in the chlorate crystals are really due to a similar 
cause, it seems much more likely that a study of the 
phenomena of the chlorate crystals will throw light on 
those of the opal, than that the phenomena of the opal 
Rothamsted ; Woburn, which, by the Duke of Bedford’s 
munificence, has become a field of experiments for the 
Royal Agricultural Society ; the stations of the Highland 
and Agricultural Society, of the National Board of Edu- 
cation in Ireland, and of the Agricultural Association of 
Sussex. It is a characteristic feature of our English 
system that the State lends no assistance to either of 
these establishments, while that of Sir John B. Lawes, at 
Rothamsted, has been conducted for many yeais on a 
princely scale by the owner at his sole cost. 
The Rothamsted station, founded in 1843, has become 
