34 The Ottawa Naturalist [May 



ihinly scattered over enormous spaces, hundreds, thousands, aye, 

 in many cases, probably millions of times the extent of our solar 

 system, and is certainly much less dense than the highest 

 vacuum we can obtain upon the earth. Notwithstanding its 

 tenuity, it is not exempt from the universal law of gravitation, 

 by which every particle of matter of every kind, in all places 

 and under all conditions, so far as we know, attracts and is at- 

 tracted by every other particle of matter by a force which is 

 directly proportional to the product of the masses of the particles, 

 and inversely proportional to the square of the distance between 

 them. The law of gravitation with which you are all familiar 

 was first enunciated by Newton and has been proved, not only 

 for all matter upon the earth, but all the motions of the heavenly 

 bodies as well as their development are governed by this law 

 with the most marvellous exactitude. You can at once see 

 what will happen when all the particles, whether solid or gas- 

 eous, of our nebula are acted on by this force, each pulling the 

 other. Mathematical demonstration proves, what is indeed 

 almost evident from common sense principles, that the resultant 

 pull on each particle will be towards the centre of the mass, 

 and this pull, although almost infmitesimally small in the 

 originally attenuated material, will tend to condense the 

 nebula. 



{Continued in next issue.) 



POISON IVY. 



By Charles Macxamara, Arnprior, Oxt. 



Before reading Mr. Gussow's interesting note in the Oct. - 

 Nov., 1911, number of The Ottawa Naturalist, few people, 

 I imagine, were aware of the existence of so many different 

 kinds of skin -poisoning plants. To the list of indigenous plants 

 of this class which Mr. Gussow mentions, I would add, though 

 somewhat doubtfully, the prickly ash (Zanthoxylum ameri- 

 canum). For while no botanical work to which I have access 

 attributes any poisonous properties to this plant, I know of 

 two cases of severe skin irritation that could not easily be traced 

 to any other cause. 



But by far the most dangerous of all our toxic flora is the 

 poison ivy (Rhus Toxicodendron). It is one of the commonest 

 and most widely distributed plants in this part of Canada, and 

 seems to thrive in almost any environment. A few years ago I 

 noticed large quantities of it growing luxuriantly on the barren 

 rock just below the Citadel at Quebec, and it is frequently found 



