1913] KNIGHT & CROCKER—TOXICITY OF SMOKE 363 
(3), according to these figures, if the smoke had not been washed, 
ammonia would have been present in the proportion of ro parts per 
million of atmosphere, or in about 0.003 sufficient concentration 
to inhibit growth. Pyridine exists in very small quantities in 
tobacco smoke, certainly far below amounts that would inhibit 
growth in experiment II (3). 
Both the nature of the response of the seedling and the con- 
centration of the smoke necessary to produce it indicate that one 
of the four carbon-bearing gases mentioned above or homologues 
of some of them fix its toxic limit. We have shown that it cannot 
be carbon monoxide on account of insufficient quantities of that 
substance. In what percentages of the smoke must the others 
exist to determine its toxicity? Let us consider experiment III (2) 
of washed smoke of the loosely rolled paper cigarette, almost as 
toxic as any tested. In this case it required 10 cc. of the smoke in 10 
liters to give NELJUBOw’s third response, or the response listed in 
the table above as “horizontal nutation and swelling.” Using 
the figures in the table above as the basis for calculation, ethylene 
must be present in 0.04 per cent, acetylene 50 per cent, and propyl- 
ene 100 per cent of the smoke, to determine its toxicity. In experi- 
ment III (2) under discussion, the heavy hydrocarbons were not 
in sufficient concentration to be detected by the gas analysis 
methods used, which should easily detect 0.2 per cent. If one of 
these three gases is responsible, it must be the ethylene. In short, 
the sweet pea seedling will give the triple response in concentra- 
tions of ethylene 0.001~-0.002 sufficient to be detected by gas 
analysis methods, while it will respond by reduced growth in con- 
centrations 0.0003 to 0.0005 sufficient to be thus detected. 
It is possible that in tobacco burned in the open, as is done 
when using it as an insecticide in greenhouses, the ammonia is 
produced in larger quantities as compared with the heavy hydro- 
carbons, and that ammonia much more nearly approaches the toxic 
limit. We have already shown that paper burned as a cigarette 
produces smoke 50 times as toxic as when burned as an open sheet. 
This means a great fall in the production of heavy hydrocarbons 
under conditions of high oxygen supply. There is probably also 
less ammonia produced when the aeration is better, for LEHMANN 
