Sept. 3, 1885] 
NATURE 
415 
symmetry is either parallel or perpendicular to the plane of 
polarisation of the light (the analyser being crossed), a few 
broad, black, curved bands crossing the main black band lying 
in the plane of symmetry, which are probably portions of the 
isochromatic curves of a very thin plate. But, on the other 
hand, some non-iridescent crystals show these bands, and some 
iridescent crystals do not show them at all. Also the iridescent 
crystals which reflect D light at moderate incidences show very 
perfectly the circular band described by Prof. Stokes (NATURE 
for April 16, 1885, p. 566, par. 9) as sharp black crescents, the 
horns of which nearly touch each other at the plane of symmetry. 
Almost the whole of the ordinary commercial crystallised 
potassium chlorate seems to consist of macles ; so that, in order 
to get a single individual crystal for examination, I have always 
had to cut away one component of a twin. 
It seemed worth while to try whether other biaxial crystals 
would, when similarly combined, give*similar phenomena. I 
took a crystal of barite (barium sulphate), the angle included by 
the optic axes of which is, according to Groth, 63° in air, and 
cut a plate of it in such a direction that the plane containing the 
optic axes made an angle of 53° with the normal to the surfaces 
of the plate. I then cut it in half and cemented one of the 
portions upon the other in a reversed position. The compound 
plate thus produced shows the secondary ellipses (which, how- 
ever, are very nearly circles) in great perfection. I have also 
made similar compound plates of borax, nitre, and citric acid, 
and found them to give similar results. H. G. Mapan 
Eton College, August 24 
The August Meteors 
BETWEEN August 4 and 20, 174 shooting stars were recorded 
here in 164 hours of observation. These included about 37 
Perseids, chiefly seen on August 5, 8, and 13, but the shower 
was not well observed owing to cloudy weather. The following 
are the chief radiant points determined from the paths regis- 
tered :— 
Radiant 
No. ate x 3 Notes 
Ie %. 16-20)... 5+ 12 ... Meteors bright, max. Aug. 20, 
2. 13 s.. (§r-58' ... Perseids: 
Bias 4-17)... 292-\52'.:. Near x Cygni: 
4. 5-13 ... 2960 ... On equator near 7 Aquile. 
5. 5-20... 317+22 ... Meteors slow and faint. 
6... 8-17 ... 318-9 ... Slow, S.W. of 8 Aquarii. 
7 ew is—l7... 325-27 +.. Slow, faint. 
8... TI-15 ... 329+8 ... Slow, bright, E. of « Pegasi. 
9 ... 16-20 ... 3450 ... Rather swift, bright. 
Io... 8-20 ... 345+53 ... Very swift, short. 
II .,, 16-20 3514+ 38 ... Swift, E. of o Andromede. 
Many other shower centres were less distinctly shown. Nos. 4 
and 9 fall exactly on the equator, and were sharply defined. 
As to the shower of Perseids on August Io, I believe it was more 
brilliant than usual, though I made no regular observations on that 
night this year in consequence of overcast sky. Many meteors were, 
however, noticed in the clear spaces which now and then occurred, 
and judging from the frequency of the apparitions the display 
was a fine one. As to the duration of the shower it was still 
visible, though very feebly, on August 20, for I registered 2 un- 
doubted Perseids during a watch of 3} hours, when 31 meteors 
were recorded. 
With regard to the minor displays of this epoch they are more 
remarkable for their number than for individual intensity. The 
most active of these radiants, as recently observed, was No. 10 
at 345°+53°, which supplied about 10 meteors, but the rate was 
less than one per hour, so that it cannot be ascribed much 
importance. W. F. DENNING 
Bristol, August 25 
Disinfection of Sewers 
In the last number of the Zawcet (August 15, 1885) I have 
read of the measures taken by the Metropolitan Board of Works 
for the deodorising and disinfecting of London sewers. Be- 
tween 30,000 to 40,000 tons of sodium manganate and from 
10,000 to 12,000 tons of sulphuric acid are daily poured in the 
London sewers. 
By what experiments has it been ascertained that the quanti- 
ties of disinfectants used are sufficient, and how is it proved that 
the sewers have been properly disinfected ? 
Ineed not point out the difference between the deodorising 
and the disinfecting of sewage. The latter may be perfectly 
deodorised, and yet be quite adapted to favour the vegetation of 
bacteria. 
The oxidising and deodorising action of sodium manganate 
cannot be sufficient to prevent bacterial life, unless when the 
salt is present in large quantities. Considering the enormous 
volume of London sewage, it is not to be believed that even 
such a vast amount of manganate as 40,000 tons per diem would 
suffice to destroy bacterial life in the sewers. 
The adding of sulphuric acid to the manganate must certainly 
enhance the disinfecting action of the latter. Only, I do not 
understand why the quantity of sulphuric acid is relatively so 
small in comparison with the quantity of manganate. I d» not 
see why manganate should be used at all when sulphuric acid, 
a more powerful and less costly disinfectant, can be used alone. 
It is well known to all who occupy themselves with the culti- 
vation and study of bacteria that these micro-organisms do not 
grow well in acid media, and that the addition of acids, espe- 
cially of mineral acids, checks their growth completely. 
It can be said that the antiseptic action of acids is of house- 
hold knowledge, for vinegar is constantly used in the preserva- 
tion of animal and vegetable products. That mineral acids 
have a greater disinfecting action than vegetable acids is also 
well known, unfortunately even by dealers in vinegar, who give 
durability to this condiment by the addition of a tiny proportion 
of sulphuric acid. 
It is probable that pathogenic bacteria, even more than the 
bacteria of ordinary fermentations and of putrefaction, are in 
need of alkaline media, and therefore are more sensitive to the 
action of acids. In the animal body bacteria invade those fluids 
and tissues where the alkaline reaction prevails; and it is 
proved that the germs of disease are easily spread by milk, 
aliquid generally alkaline. Moreover, it has been proved by 
experiments on some pathogenic bacteria that gastric juice, 
although of so slight acidity, easily, and sometimes effectively, 
checks their development. 
Sewage contains all the elements necessary for the nourish- 
ment of bacteria, and its alkaline reaction renders it very favour- 
able to their growth and preservation. Disinfection means the 
destruction of existing bacteria and preventing the development 
of newly-sown bacterial germs. Therefore Iam persuaded that 
the cheapest and more simple method for effectively disinfecting 
sewage is to render its reaction permanently acid by the addition 
of a sufficient quantity of mineral acid. 
There are of course disinfectants far superior to mineral acids 
in antibacterial energy. But they are generally costly substances, 
that cannot be applied to the disinfection of such an enormous 
quantity of matter as the sewage of a town. As for cheap dis- 
infectants, such as ferrous sulphate, ferric chloride, sodium 
manganate, their action is inferior to that of mineral acids, 
Especially of the two former it can be said that their deodorising 
action is due to their saline constitution, and their disinfecting 
action to their acid reaction. 
The great difficulty in extensive disinfections is to ascertain if 
the disinfection has been complete—se. if the substance disin- 
fected has been rendered unfit for the development and preserva- 
tion of bacteria. Even laboratory experiments, to ascertain the 
minimum of disinfectants necessary for the destroying of 
bacteria, are not easily conclusive. But, in using acids, the 
disinfection can be considered complete when a permanent acid 
reaction is obtained. 
I do not believe the quantities of sulphuric acid poured in the 
London sewers sufficient to give a permanent acid reaction to 
the sewage. Disinfection must be done completely, or not at 
all: there are no half measures in disinfection. ‘Therefore I 
maintain that the London disinfection is useless, and the sewage 
remains likely to become the culture fluid of infectious germs, 
unless the sewage is rendered permanently acid. All the sodium 
manganate added to a sewage that remains alkaline, gets de- 
composed ; the manganese precipitates as sulphide, or is 
deposited in combination or mixture with the organic sediment. 
The sewage will thus be cleared and deodorised for a while ; but 
it still contains in solution all the elements necessary for the 
nourishment of Bacteria, and is still favourable to their growth 
and preservation. The disinfecting action of sodium manganate 
would avail only if large quantities of the salt remained dissolved 
in the sewage, over and above of the quantities decomposed in 
deodorising and clearing the putrid fluid. 
It might be objected that, even if mineral acids stop the 
