266 LIGHT SCIENCE FOR LEISURE HOURS. 



'consists of what has passed through the stomachs 

 of animals,' or has suffered decomposition in some way 

 or other. This unpleasing matter, like the * molecular 

 activity,' floats about at a height of five feet, or there- 

 abouts. 



After this, one begins to recognise the manner in 

 which some diseases propagate themselves. What had 

 been mysterious in the history of plagues and pesti- 

 lences seems to receive at least a partial solution. 

 Take cholera, for example. It has been shown by the 

 clearest and most positive evidence that this disease is 

 not propagated in any way save one that is, by the 

 actual swallowing of the cholera poison. In Professor 

 Thudichum's masterly paper on the subject in the 

 'Monthly Microscopical Journal,' it is stated that 

 doctors have inhaled a full breathing from a person in 

 the last stage of this terrible malady without any evil 

 effects. Yet the minutest atom of the cholera poison 

 received into the stomach will cause an attack of 

 cholera. A small quantity of this matter drying on the 

 floor of the patient's room, and afterwards caused to 

 float about in the form of dust, would suffice to prostrate 

 a houseful of people. We can understand, then, how 

 matter might be flung into the streets, and, after 

 drying, its dust be wafted through a whole district, 

 causing the death of hundreds. One of the lessons to be 

 learned from these interesting researches of Mr. Dancer 

 is clearly this, that the watering-cart should be re- 

 garded as one of the most important of our hygienic 

 institutions. Supplemented by careful scavengering, it 



