152 Answers to Queries. 
370.—To Increase Specific Gravity.—On reading my own 
answer in the July number, it occurred to me to try practically 
whether my theoretical advice would arrive at a practical result. 
This I knew must be the case if the tables were correct from 
which the data were taken. The strongest solution of ammonia 
I had was labelled 880, but had an actual gravity of 884, which, 
according to the table, contained 36 per cent. of ammonia. This 
was to be reduced toa gravity of 891, or a solution containing 
32°8 percent. Following out my own instructions in the manner 
most would have done, I find that 914 parts of solution of sp.gr. 
884 contained the exact amount of ammonia required by the table 
to make a gravity of 891. Then, measuring out g1§ cubic cen- 
timetres of solution, 884, I made it up to 100 parts. ‘The specific 
gravity of this was not 891, but 893. On thinking over the 
matter, I saw that my parts should not be measured, but weighed. 
Now, as the gravity of the ammonia is 884, it will be necessary to 
take more than g1j parts by measure to obtain the weight of 
ammonia required. too cc. of ammonia solution contain only 
31°82 grammes of ammonia instead of 36 grammes or 884 multi- 
plied by 36 and divided by ro00. So also 100 cc. of solution of 
gravity, Sgr, contain 29'228 grammes instead of 32°8, so that we 
must by measure take that volume of ammonia of 884 gravity, 
which contains 29'228 grammes, instead of that which contains 
32°8. This amount is 92 cubic centimetres instead of 913. In 
the next experiment I therefore took g2 cc. of ammonia solution, 
884, and made it up with water to roo cc. The gravity of this 
was 8913, or sufficiently near for the purpose. y: Woe 
372.—Microscope.—Your correspondent had better procure 
** How to Choose a Microscope,” by ‘‘ A Demonstrator,” or “The 
Student’s Handbook to the Microscope,” by ‘ A Quekett Club- 
man,” either of which will afford much useful information on the 
choice of the instrument. Baker’s ‘‘ Educational Microscope” at 
43 3s. is the cheapest one that would be of much use. Better 
get a good stand, with inch and }inch powers, costing about six 
guineas ;. to this, accessories can be added if wanted. A botani- 
cal simple microscope is useless except for dissecting purposes. 
: GH. B; 
372.—Microscope.—It is impossible to give a complete answer 
to R. C., as he does not say for what purpose he wants a micro- 
scope, and even if he gave this, there are many different opinions 
on the matter, as everyone has his idiosyncracies and prefers the 
instrument to which he is accustomed. 
However, unless R. C. wants an instrument for the elementary 
work of botany or for dissecting purposes only, it may be safely 
stated that a simple microscope, such as ‘‘ Henslow’s,” would not 
satisfy him. He ought to get a compound microscope—of course, 
