January 28, 1897] 
NATURE -..< 
“ 293 
LETTERS TO THE ELVITOR. 
(The Editor does not hold himself responsible for opinions ex- 
pressed by hts correspondents. Netther 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.) 
The Oyster Question. 
Pror. THORPE’S allusion to myself in connection with oysters 
and their ways in a recent’ number of NaTuRE (p. 106) has 
reminded me that I also have something to say on the subject, 
a propos of the ‘Yellow Book” issued by the Local Govern- 
ment Board. 
First, I wish to state that several references to my work, made 
in that and other recent publications on oysters and disease} ought 
rather to be to the work of my colleague Prof. Boyce, or to our 
joint work. 
In conjunction with Prof. Boyce, I have published three notes 
on the subject, and the bacteriological parts of these have, 
naturally, been contributed by my colleague. The first note was 
read (and circulated) at the Ipswich meeting of the British 
Association in September 1895, the second was brought before 
_ the Liverpool Biological Society in January 1896, and was pub- 
lished that same month in the Annual Report of the Lancashire 
Sea Fisheries Laboratory for 1895, while the third was read (by 
Prof. Boyce) to Section I at the Liverpool meeting of the British 
Association last September, and reprints have since been circu- 
lated. The points that we believe we have demonstrated (I do 
not say that they were all new when announced—some were 
known, others suspected, some denied; but I think we have 
given definiteness to all) are as follows :— 
(1) The beneficial effect of free change of water round the 
oysters. 
(2) The deleterious effect of keeping the oysters in stagnant 
water. 
(3) The considerable toleration of sewage shown by the 
oyster, and its power of absorbing large quantities of faecal 
matter. 
(4) The great increase (e.g. from 10 colonies to 17,000 per 
sample) in the bacterial contents of the pallial cavity and of 
the rectum when the oyster is laid down in close proximity to 
the mouth of a drain. 
(5) The presence of more bacteria in the pallial cavity than 
in the alimentary canal of the oyster. 
(6) The fact that the typhoid bacillus does not flourish in 
sea twater. There is no initial or subsequent multiplication ; 
on the contrary, it seems to die off very rapidly as time in- 
creases after inoculation. ‘ 
(7) The fact that the typhoid bacillus does not multiply in 
the stomach or tissues of the oyster. 
(8) The presence of a “‘ pale green” disease, characterised by 
a leucocytosis, in certain oysters. 
(9) The fact that the dark blue-green colour of the Marennes 
oysters has nothing to do with copper. 
(10) The fact that perfectly fresh oysters contain fewer 
bacteria than those that have been stored or kept in shops. 
(11) The enormous number of the common colon bacillus 
present in very many oysters obtained from shops. 
(12) The possibility of getting rid of bacterial infection by 
placing the oyster in a stream of running water. There is a 
great diminution or total disappearance of the B. typhosus 
under these circumstances in from one to seven days. 
Perhaps it is on the last of these conclusions that Prof. Thorpe 
has founded his remark, that the oyster has confided to us its pre- 
ference for clean water. Whatever it may frefer, Dr. Bulstrode 
has abundantly demonstrated in his report, that the oyster is not 
always found in clean water; and the practical conclusion 
of all these investigations and reports ought to be the 
enforcement of the two sanitary measures which Prof. Boyce 
and I recommended a year ago, viz. ‘‘(1) the strict examination 
of all grounds upon which oysters are grown or bedded, so as to 
ensure their freedom from sewage, and (2) if practicable, the use 
of “ dégorgeoirs” in which the oysters should be placed for a 
short time before they are sent to the consumer” ? (“‘ Rep. 
Lanc. Sea-Fish. Lab.” 1895, p. 72). 
1 This letter was written before the Christmas vacation, but has been de- 
layed by examination and other engagements. 
2 Probably the most satisfactory method for all concerned—producers, 
customers, and sanitary authorities—would be to have all oyster beds, parks, 
layings and ponds inspected and “ licensed,"’ and to have no oysters ex- 
posed for sale except such as come from a certified locality. 
NO. 1422, VOL. 55] 
; common with the dark blue-green of 
yl am interested to see that Dr. Bulstrode (in the Local 
Government Board Report) independently corroborates our 
discovery of a pale green disease in some relaid oysters in this 
country. This is especially important, since Dr. Carazzi, of 
Spezia (whose results differ from those of most other investi« 
gators of molluscan structure and physiology), in a recently 
published paper, has doubted the existence of this green disease 
—probably becausé he has never met with it. He has drawn 
his conclusions mainly from the normal green Marennes oyster. 
We distinctly stated that’ the pale green disease had nothing in 
the ‘‘huitres de 
Marennes,” and that we regarded the latter as being healthy 
and normal. 
Ii is evident, then, that there are several distinct kinds of 
greenness in oysters. All recent investigators are agreed (except, 
possibly, Dr. Carazzi—I cannot venture to answer for him) 
that the green colour of the Marennes oyster has nothing to 
do with copper. Prof. Boyce and I have shown, and Dr. 
Bulstrode supports it, that the green of the (¢.g. Fleetwood) re- 
laid American oyster is due to a disease or leucocytosis,, while 
now Prof. Thorpe tells us that (as was originally supposed, and 
then doubted) the greenness of the Falmouth oyster is really due 
to copper. W. A. HERDMAN, 
Liverpool, January 9, 1897. 
P.S.—In connection with the correspondence which has taken 
place in NaTuRE, since Prof. Thorpe’s article, I am glad to be 
able to add my testimony to that of Dr. Cartwright Wood and 
others as to the purity and healthy state of the Pyfleet oyster. I 
have visited the locality, have seen the oysters dredged up, and 
have examined (both biologically and gastronomically) many 
specimens, with entirely satisfactory results. 
The Symbols of Applied Algebra. 
I am glad to see that attention is being forcibly drawn to the 
value and importance of considering the symbols in physical 
equations as primarily denoting quantities, and not mere 
numerical multiples of some unincluded standards. The latter 
mode of considering them, though often practically convenient, 
is entirely subsidiary, and a deduction from the primary equa- 
tions between the quantities themselves. 
The equation w = mg is a special case of Newton’s second 
law ; it represents a fact of nature, and has nothing to do with 
systems of units. It ds true in any units :—e.¢. 
981 dynes = I gramme weight 
= I gramme-mass x 32°18 ft./(sec.)”. 
The curious discussion about so simple an equation is kept up 
by those who wish to make all equations numerical only. Todo 
this they must have a system of standards or units which them- 
selves satisfy the equation. The numerical coefficients will then 
also satisfy the same equation, and the standards or units may 
be cancelled or omitted. The metric system has acquired the 
desired units by the invention of the dyne; and to do the 
same for the British system requires one of three alternatives :— 
(1) To take as unit of mass 32°18 Ibs. or (say) a ‘‘ perry,” instead 
of 1 lb. ; (2) to take (32°18 ft. /(sec. )”) as unit acceleration ; or (3) 
to employ a special unit of force, based directly upon Newton's 
second law, and upon the pound, the foot, and the second. 
Any of these conventions will serve: they are only needed for 
arithmetical interpretation of the equation, and, of them, the 
last is, on the whole, the simplest for general application, be- 
sides being in accordance with the universally adopted metric 
convention. ALFRED LODGE. 
Coopers Hill, January 18. 
as connecting the 
D 
weight, volume, and specific gravity of a body. Does he 
seriously suggest that this is ‘‘independent of every system of 
units”? Surely it requires that the unit. of weight should be 
the weight of unit volume of the standard substance. Would 
he give this formula to a student who measured forces in 
poundals ? ny bt 
The formula neatly illustrates the objection to the poundal. 
The C.G.S. system is theoretically perfect; the system in 
which the pound is the unit of force is, no doubt, theoretically 
objectionable, but is practically extensively used. 
Pror. LopGE gives the formula s = 
