Ventilation with Heating. 75 



not having a temperature sufficiently high to enable it to 

 compete successfully in the race to the ceiling with the warmed 

 fresh air. Some of the respired air will be caught up with the 

 stream of products of combustion of the gas jets, and may escape, 

 but for so far there does not seem to be any place where the 

 ventilation is so abundant that more than a fraction of the gas 

 products are got rid of This system of ventilation resembles in 

 its wastefulness the plan of grazing cows in a field, where they 

 trample and befoul more than they consume. It is for this 

 reason of wastefulness that writers on ventilation demand such 

 a large amount as 2,000 cubic feet of air per hour to make a 

 healthy atmosphere for a man ; — this air is simply mixed with 

 all the air in the chamber, and dilutes its poison down to a point 

 that is not perceptibly offensive ; — while for an hospital with sick 

 persons, 6,000 cubic feet per hour is required for each patient. 



The purity of the air of a room is determined by the propor- 

 tion of carbonic acid present. The air becomes oppressive 

 when the acid is above 1 5 per ten thousand. But this gas is not 

 the worst foe to dread. The vapours given off by the lungs and 

 from the surface of the body are more dangerous. These vapours 

 do not diffuse with the rapidity of the carbonic acid. They 

 require a current of air to disperse them. Carbonic acid, although 

 it diffuses, yet, as Dr. Letts on a former occasion showed to us, 

 will remain in a vessel for a time with a definite surface like a 

 fluid. There is reason to believe that the respired air and the 

 emanations from the body may tend, if undisturbed, to form a 

 stratum in which an assembly in a crowded room is bathed. 

 The respired air given off is so rapidly mixed with the surround- 

 ing cold air, and is so loaded with vapour and carbonic acid, 

 both these combined being heavier than pure air, that it seems 

 to have little tendency to ascend, as can easily be tested on a 

 calm day, when it is possible to have the pleasure of sharing in 

 the pipe of the gentleman walking some twenty yards in front, 

 or better still, in his last glass of whiskey ! On a frosty day 

 when from its warmth the tendency of the breath to rise should 

 be greatest, it may be seen streaming from a horse's nostrils as 



