DUST AND DISEASE. 317 



can breathe without annoyance in a space so crammed 

 with smoke, that a single inhalation without the respirator 

 would be intolerable. I wrote to the chief officer of the 

 Metropolitan Fire Brigade, asking him whether svich a res- 

 pirator would be useful. He replied to me that it would, 

 but added that he was aware of every invention of the kind 

 in all the countries of Europe, and that none had been 

 found of any use. At my invitation he was kind enough 

 to come to the Royal Institution with two firemen and an 

 assistant. The three latter, wearing such respirators, went 

 in succession into the smoke-filled space, and on returning, 

 stated that they had not experienced the slightest discom- 

 fort, that they could have remained there all day long. 

 Captain Shaw himself repeated the experiment with the 

 same result. I am confident that sooner or later this res- 

 pirator will be employed, to the great benefit of a class of 

 men whose actions in critical circumstances I have often 

 had occasion to admire. 



Application of Luminous Beams to Water. 



The method of examination here pursued is also appli- 

 cable to water. It is in some sense complementary to that 

 of the microscope, and may I think materially aid inquiries 

 conducted with that instrument. In microscopic examina- 

 tion attention is directed to a small portion of the liquid, 

 and the aim is to detect the individual suspended particles. 

 By the present method a large portion of the liquid is il- 

 luminated, its general condition being revealed, through 

 the light scattered by suspended particles. Care is taken 

 to defend the eye from the access of all other light, and, 

 thus defended, it becomes an organ of inconceivable deli- 

 cacy. Indeed, an amount of impurity so infinitesimal as to 

 be scarcely expressible in numbers, and the individual par- 

 ticles of which are so small as wholly to elude the micro- 



