CHEMISTRY 243 



optical prism bottles. Isinglass with a little acetic acid answers 

 very well in most cases. 



130. Test-tubes. Very often a small test-tube answers 

 all purposes as well as any tank, besides being cleaner and 

 using less material. The rather flattened form of test-tube 

 sometimes available is, for obvious reasons, better than circu- 

 lar tubes ; but even in the latter any ordinary reaction can be 

 exhibited perfectly well. 



131. Experiments. It is perfectly useless to attempt a 

 list of experiments in detail, but a few representatives may be 

 given, with one or two practical hints. 



In precipitations it will usually be found expedient to 

 employ solutions more than usually diluted, as the faintest 

 opacity is projected on the screen with a conspicuousness that 

 would hardly be credited without trial. Suppose, for instance, 

 we want to exhibit the precipitation of silver from solution of 

 its nitrate by hydrochloric acid. It will be found that this is 

 better shown by a weak solution of one, two, or three grains 

 per ounce ; and anyone who has never before attempted this 

 method will be astonished at the apparently dense masses of 

 cloud which will be formed in such a solution, by a single 

 drop of the acid introduced at the tip of a glass rod. 



In reactions involving liberation of gas the same remarks 

 apply. Minute bubbles are so magnified, and appear so black 

 upon the screen, that solutions adapted for giving what would 

 be called very slight reactions, or often a mere trace, give the 

 best results. This fact makes the lantern method peculiarly 

 useful for exhibiting reactions involving the more expensive 

 chemicals. Such an experiment as the formation of calcium 

 carbonate by blowing carbonic acid into clear lime-water, is 

 very much more impressive projected, than when shown in any 

 other way. 



The usual test-reactions are very readily shown, in tanks 

 of litmus or other test solutions ; but these are better made 

 pretty strong, in contradiction to the preceding, in order to 



