STUDIES ON CONTACT INSECTICIDES. 11 
boiling point of 106° C. is considerably more toxic than the nearly- 
related pyridine, which has a boiling point of 115° C. Paraldehyde 
with a boiling point of 124° C. has only about one- third the toxicity of 
chlorobenzene, whose boiling point is but 8° higher. Pyrogallol, 
aniline, and toluene have closely similar toxicities, but widely diver- 
gent boiling points. A study of Table 2 will reveal other discrepancies. 
Tattersfield and Roberts {13), in a recent study of the effects of vapors 
of organic compounds on wire worms, conclude that while " there is a 
fairly close relationship between toxicities and the vapor pressures, 
rates of evaporation, and volatilities of compounds of the same 
general type," lethal effects are often directly determined by chemical 
constitution. Compounds with irritating vapors, such as allyl 
isothiocyanate, chloropicrin, and benzyl chloride, gave toxic values 
which were not closely correlated with their vapor pressures or rates 
of evaporation. It was also found that isomeric compounds having 
similar boiling points sometimes differ widely in toxicity. 
There can be no question of the importance of volatility as a factor 
in the toxicity of both contact insecticides and fumigants. But the 
toxicity of a chemical appears frequently to depend upon properties 
other than volatility. High toxicity, therefore, may occur in com- 
pounds like chloropicrin and hydrogen cyanide, which have high 
volatility, or in nicotine and tetramethylammonium chloride, in 
which the volatility is low or almost nil. 
TOXICITY AND CHEMICAL STRUCTURE. 
The addition of hydrogen atoms to the cyclic nucleus has a marked 
effect on the toxic activity of some compounds. Piperidine (hexa- 
hydro pyridine) has about five times the toxic activity of pyridine. 
Cyclohexane (hexahydrobenzene) is more toxic than benzene, but 
the increase is not so marked as that observed for piperidine. On 
the other hand, quinoline and tetrahydroquinoline are of approxi- 
mately equal toxicity. Hydrogenation of the pyridine nucleus of 
nicotine to form hexahydronicotine greatly reduces toxicity, the 
lethal concentration for hexahydronicotine being about ninety times 
J that of nicotine. The addition of hydrogen atoms to the ring, there- 
fore, may either increase or decrease toxicity. It should also be 
noted that although the changes in toxicity resulting from the 
addition of hydrogen may be considerable, the differences in boiling 
point between parent and hydrogenated compound are small. 
In a number of instances it was noted that the toxicity of homo- 
logous compounds increased as the series ascended. This is particularly 
well shown in the series, benzene, toluene, xylene, and in the aliphatic 
alcohols, methyl, ethyl, normal propyl, normal butyl. The phenol 
series, phenol, resorcinol, pyrogallol, does not show this relation, 
pyrogallol having only about one-half the toxicity of resorcinol. 
i This is contrary to what has been observed in higher animals, in 
which an increase in the number of OH groups in the benzene ring is 
generally accompanied by increased toxicity (1 , p. 29). The 
trimethylamine and triethylamine hydrochlorides and the tetra- 
methylammonium and tetraethylammonium chlorides are also 
exceptions, for in each case the lower members (methyl compounds) 
are the most toxic. Tattersfield and Roberts (13) have also recorded 
increased toxicity in successive members of homologous series of 
