188 Scientific Intelligence. 
3. For the hardest crayons, twenty seven grains to one ounce ; and 
4. For those which are to leave brilliant traces, thirty six grains of 
isinglass, (ichthyocolla,) to one ounce of hematite. 
€ gum, or isinglass, is dissolved separately, and passed through 
a woolen strainer ; the powder is then added, and the mixture is 
placed over a moderate fire, until the mass acquires a suitable con- 
sistency to bear rubbing in a mortar of porphyry, to render it homo- 
geneous. ‘To form the crayon, the paste is forced through a cylinder, 
and the sticks are dried and divided into pencils two inches long. 
They are then pointed, and the crust, formed during desiccation, is 
removed.—Idem. VT. 294. 
2. On the Preservation of Potatoes. (Aun. Soc. d’Agric.)— 
Potatoes at the depth of one foot in the ground, produce shoots near 
the end of spring ; at the depth of two feet, they appear in the mid- 
dle of summer ; at three feet of depth, they are very short, and ne- 
ver come to the surface ; and between three and five feet, they cease 
to vegetate. In consequence of observing these effects, several par- 
cels of potatoes were buried in a garden, at the depth of three feet 
and a half, and were not removed till after intervals of one and two 
years. They were then found without any appearance of germina- 
tion, and possessing their original firmness, freshness, goodness and 
taste.—Idem. 
MEDICAL STATISTICS. 
1. In a work entitled “ Elements of Medical Statistics: &c.” by 
F. Bisset Hawkins, M. D: &c. London, 1829, the following facts are 
Stated in a review of this work, in the Quar. Journal. 
The mean term of Roman life, was thirty years—that is to say; 
taking one thousand persons—adding together the years they indi- 
vidually attained, and dividing the total by the number of persons, 
the result is thirty. It is curious to contemplate the improvements 
which have been effected in the course of time in the value of hu- 
man life. Mr. Finlayson has ascertained, that in England, at the 
present time, the expectation of life, for persons similarly circum- 
stanced, is at least fifty years, giving us a superiority of twenty years 
above the Roman citizen. The mean term among the easy classes 
at Paris, is at present forty-two. At Florence, to the whole popula- 
Hon, it is still not more than thirty. 
