Vicias on Frcnebular Oondltloyis. — .1. Winc/ieU. 199 
work, lu' cxpliiiiKMl with considerable detail, his conception of 
the mode of aggregation of "cosmical dust" into nebular and 
cometary assemblages." In this he spoke in ])articular of the 
necessary collisions of constituent parts, the development of 
intense heat, and the furmation of an elongated train, in the 
case of comets moving within the Solar System. '- 
Tins theory of the origin and constitution of comets has 
been heartily endorsed, and discussed with ample learning, in 
a series of papers published by Prof. Lockyer, in Nature, in 
volumes xxxvii, xxxviii, xxxix, but more specifically in the 
latter volume. Here (page 402), he mentions professor Tait, 
and without quoting, explains his views. He says professor 
Tait's researches have not been published in e.Henso^ but a 
summary of results appeared in ''Good "W'ords" "some time 
ago.'"' These Mr. Lockyer presents in language from which 
the following are extracts : "In the case of comets of but 
small masses, the component materials would be small and 
far apart. * * * While the swarm which builds up the 
to explain the formation of worlds antagonistically to the Kantian con- 
ception, while that of the pre.sent writer was to gain a starting-point 
for the nnfolding of that conception. Sir William Thomson is said to 
to entertain the opinion that the "origin of the planets was through a 
gradual accretion of meteoric matter." 
""World-Life," 1883, pp. 71-75, 482-;5. 
'-'It seoms necessary, as will api)ear, to make special citations from 
the work mentioned. After speaking of the aggregation of ncbulfe, 
tlie author continues: "In the nearer neighlwrhood of some great at- 
tractive centre, the velocity of one of these swarms is accelerated. Its 
form becomes more elongated. The internal movements of the parts 
becom»' more vigorous, ci^llisions are sharper, and Hashes of hght are 
evolved, and the posterior train is expanded. Further influence 
exerteil hy the central boily increases all those conseciuence.s. The 
head of the swarm becomes permanently luminous. The long gather- 
ing swarm is n<jw a comet" (p. 75). The contingencies happening to a 
cometary aggregation once introduced into our system, are next traced, 
"A cannon-ball moves 1400 to 2000 feet in a second, and yet its impact 
upon a solid body always develops a flash of light. But" this velocity 
is mere rest when compared with that of a comet in it.s flight. Now in 
case of these mutual collisions among the parts of a comet, the veloci- 
ties of some will be accelerateil, and those of others retarded. Those 
retarded are liable, of course, to be acceleratetl again by other collis- 
ions, so that the total amount of motion in the assemblage should 
remain constant, so far as actions in the system are concerned. Never- 
theless, the changed velocity of a part results in a change of intensity 
of action from without," etc. Thus, not to quote at length, the comet 
tary aggregation is i)artly pulverized, an<l grailually torn asunder — 
disintegrating into the meteoroidal stage (pp. -I82-.3). These principles 
are applied to the slow contraction of Saturn's rings, and to the zodia- 
cal light. 
'^From Dr. CroU the writer learns that this was in tbo volumt' for 
1875, p. 801. The date of publication does not appear. 
