PHYSIOLOGICAL DEMONSTRATION 241 



clearly projected on the screen by a power of about 1,200 

 diameters. 



I must content myself with these representative experiments 

 in a branch of demonstration witli which I can necessarily have 

 little practical acquaintance ; but they will suffice to indicate 

 to those who are familiar with the subject, the applicability 

 to it of the projection method. That method will be found 

 more and more convenient, by reason of the number who can 

 simultaneously receive the same instruction, the more it is 

 practised. 



CHAPTER XVI 



CHEMISTRY 



OP course the only chemical experiments adapted for public 

 projection are of the qualitative, and most of them of the 

 ' popular ' class. Of that scrupulously accurate quantitative 

 work which forms so large a part of modern chemistry, the 

 lantern can take no cognisance. It can only impart a popu- 

 lar knowledge of those general phenomena which comprise 

 what the majority of people understand as chemistry; but 

 even this has its value as a part of general education, and 

 as giving some intelligent understanding of a great deal that 

 goes on around us. 



129. Tanks. The major part of such experiments may 

 be classed under the general head of 'reactions.' Nearly all 

 of these which give any conspicuous phenomena, are better 

 shown in the lantern, and with the advantage of employing a 

 great deal less weight or bulk of material. The apparatus 

 most frequently required will be some sort of tank, with 

 parallel glass vertical sides, whose contents will exhibit on a 

 small scale the desired reaction, and which takes the place 



B 



