Exchange. 39 
renewed, and the same objection applies to bisulphide of carbon, 
which, however, is excellent for destroying mites if they find their 
way into the collection. ‘Then there is carbolic acid, which seems 
fairly satisfactory. Also, there is the plan of poisoning the 
specimens, with corrosive sublimate, which, however, must be 
done when the insects are being set, not after, otherwise the 
specimens get relaxed by it, and care must be taken not to let the 
poison get near the pin, which it will corrode. ‘Thus it appears 
that for specimens not poisoned when set, camphor is the only 
preservative that will last for any length of time, but this is open 
to the serious objections just mentioned, to which may be added 
the fact, that grease produced by camphor is very difficult to 
dissolve. Can your readers suggest any way out of the difficulty ? 
Have carbonate of ammonia or sulphur been tried, and if so with 
what success ? Go: 
329.—Tracing Paper,—Will any of your readers tell me how 
tracing paper is made ; what kind of paper is used; what kind of 
varnish, and the cost of it ; how it is to be applied, and how dried ? 
ANXIOUS. 
330.—Salt.—Will any physiologist tell us in plain terms why 
salt causes thirst? What is the relation between the action on the 
stomach and intestines and the parched tongue ? Zs 
30l.—Canada Balsam.—What are the various merits of 
Canada Balsam diluted with (1) Benzole, (2) Turpentine, (3) 
Chloroform respectively ? Are any of these better than or as good 
(except for the saving of trouble) as balsam thinned by heat? Z, S. 
332.—Blood Corpuscles.—What is the use of the colourless 
blood corpuscles ? Kindly also point out their morphological and 
physiological analysis, especially referring to Amoeba, Torula, and 
various species of Bacterium. By O? B: 
333.—Lobster.—What evidence do we possess that the green 
gland of a lobster fulfils the office of a kidney ? Ba@: 5; 
334.—Octopus—I have a slide marked ‘Odontophore of 
Octopus.” Will someone tell me if the animal possesses the 
usual lingual ribbon common to the Mollusca ? 1s ea is 
Exchange. 
Wanted, Parasites, unmounted, prepared. Will give Slides (Anatomical, 
Pathological, Botanical, Micro-fungi, Parasites, Zoophytes, etc.) in exchange. 
Splendid Slide of Bacilli in section of Lung (acute miliary tuberculosus). 
Home and foreign correspondents wanted to exchange slides or micro material 
of above or other subjects.—Fred. Lee-Carter, Gosforth, Newcastle-on-Tyne. 
