268 LIGHT SCIENCE FOR LEISURE HO.URS. 



6 consists of what has passed through the stomachs 

 of animals,' or has suffered decomposition in some way 

 or other. This unpleasiiig matter, like the e 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 regarded 

 as one of the most important of our hygienic insti- 

 tutions. Supplemented by careful scavengering, it 



