SCIENTIFIC SUMMAEY. 
647 
practice, the exclusion of less than 20 per cent, of the seed-wheat will 
probably be sufficient to insure this result. 3. The process of selection of 
seed by density is easy and inexpensive ; in some cases, however, it might 
advantageously be left to the seedsman, j 4. An acclimatized seed- wheat 
yields a heavier crop than wheat grown under different conditions of climate, 
soils, &c. 
In what parts of plants does Potash reside ? — On this subject some curious 
chemico-agricultural inquiries have recently been reported to the French 
Academy by M. Isidore Pierre. The object of this savant's researches was to 
discover in what parts of cereal plants, and during what seasons, potash is 
most abundant. In carrying out his investigations, the greatest care was taken 
to insure the examination of the corresponding parts of several specimens of 
com. The plants were analyzed before flowering, in bloom, and in the fruit- 
time. From a great number of experiments, the author draws the followiug 
conclusions : — 1. That in the various parts of the plant (leaves, nodes, and 
internodes) the proportion of potash to soda increases in a well-marked 
manner as we pass from the lower to the upper parts. 2. That in parts 
of the same name and position this proportion tends to diminish as the plant 
advances toward the period of ripening. Sometimes this diminution is much 
less marked in the leaves than in the nodes and internodes. It seems that 
potash salts play a more important part in the life of the plant than soda 
salts : in fact, it is seen that the former predominate in those parts of the 
plant which may be considered the most perfect in the last-developed portions, 
whilst soda is generally found most abundant in the structures which are 
earliest developed, and whose office is rather temporary, or at least of secondary 
importance. From these facts, it will be seen that the adversaries of the 
employment of common salt as a manure have a new argument in their 
favour, so far as relates to the cultivation of corn. The nodes of cereals con- 
tain an enormous quantity of potash, more than 4‘5 per cent, of their entire 
weight, and nearly half (42 ’5) of their ashes. 
4 . 
ASTRONOMY. 
Cometary Systems. — M. Hoek, of Utrecht, has discussed the cases among 
the 65 comets which have appeared since 1844, where the aphelion positions 
do not surpass an angular distance of ten degrees. Among the ten instances 
which he has detected where two or more comets come within those limits, 
he draws especial attention to the case of the comet III. of 1860, and those 
of I. and VI. of 1863, which were at an angular distance of only three 
degrees. This proves, he considers, that some comets have on different 
occasions been near each other at a great distance from the sun. He proceeds 
to calculate the dates when those three comets were at nearly the same 
distance from the sun, which he finds to be in the years a.d. 757 and 1021, 
when they were within 600 and 500 times the length of the radius of the 
terrestrial orbit, which he thinks proves that they had not been brought to- 
gether by chance, but that they once composed separate systems in space. 
From those data he comes to the conclusion that systems of comets exist 
