PUTREFACTION IN BLOOD. 61 



putrid matter ; and to throw light on them is the object of 

 further inquiries. Metallic salts by no means remove them 

 when formed^ although they may prevent their formation 

 by obstructing decomposition. But metallic salts remove 

 the sulphuretted hydrogen, it is therefore shown that this 

 gas is no essential element in a putrid odour ; it is even 

 probable that it may tend to diminish its virulence. 



The question as to the physical conditions of these sub- 

 stances presses for an answer. I have pictured them to 

 myself in some cases as particles hollow or otherwise. 

 When we make hydrogen from water by zinc and sulphuric 

 acid, a number of particles rise from the bubbling liquid ; 

 some fall back, others float onward. At a considerable 

 distance we smell hydrogen ; but it is not only the gas we 

 smell, we become sensible of a considerable irritation, and 

 of sulphuric acid floating in the air. When a bubble rises, 

 it seems to take along with it a little of the acid in the 

 form of a soap-bubble, and floats like one. It is extremely 

 probable that when it has floated long it loses the vesicular 

 state, and becomes concrete, as De Saussure terms such a 

 drop ; at least we might expect this from the constitution 

 of such a globule. I should suppose the same thing to 

 occur in all rapid evolutions of gas, this necessarily de- 

 pending much on the tenacity of the liquid. If minute 

 vesicles rose, they would become liquid globes after a time ; 

 and if these, by evaporation, lost their Hquid, they would 

 leave in the atmosphere solid shapeless particles, perhaps 

 such as the microscope reveals. This may be one method 

 by which substances become coated over with putrescible 

 matter. That the substances are carried into the air we 

 have other proofs, not even forgetting the despised one, 

 that some insects live in the vapour, and seem to obtain 

 no other food. Wherever many flies are found, there is 

 to be found a large amount of matter affecting the sense of 

 smell ; and great cleanliness causes them to leave a house. 



