Proceedings of the Polytechnic Association. 803 



their cables, and three of them were driven on shore. Aside from 

 the question of safety, the comfort and rest of passengers is secured 

 while the ship is riding at anchor during rough weather. 



Distillation of Petroleum. 

 Dr. Joseph Hirsh, of Chicago, in a communication to The London 

 Chemical News^ gives the following rules, which, by experience,* are 

 found to regulate the distillation of a mixture of native hydrocarbons 

 on an extensive scale : 1. The difference of temperature between the 

 actual boiling point of oil of definite gravity and of the temperature 

 to which it is raised, is proportionate to the effect of the process of 

 " cracking ;" i. e., the more the temperature of the actual boiling point 

 of oil of definite gravity is above the temperature to which the same 

 oil is raised, the greater is the quantity of light oil obtained. If, then, 

 we wish to reduce the gravity of a heavy oil greatly, we should have 

 to employ an exceedingly low temperature, so low, sometimes, as to 

 suspend actual distillation for a short time. 2. The gravity of the 

 distillate, resulting from the reduction of temperature, will be directly 

 proportionate to the said reduction ; i. e., if we reduce the temperature 

 to a degree at which naphtha of 0.700 boils, the resulting distillate 

 will possess a specific gravity of 0.700, regardless of the gravity of the 

 original oil. This law enables us to produce a distillate of any desired 

 gravity (below that of the oil before distillation) from any crude oil, 

 and a due regard to it enables us to produce illuminating oil, without 

 great quantitative loss, from the light Pennsylvania oils, of the same 

 specific gravity as that produced from the heavier Ohio, Canada or 

 California oils. In distillation, the temperature, therefore, should 

 always be reduced to the boiling point of oil of the specific gravity 

 desired. 3. The difference between the temperature of the two boil- 

 ing points, viz., of the oil being subjected to distillation, and of the 

 desired distillate, is in direct proportion to the height of the still 

 employed, or, which produces the same effect, to the facility for cool- 

 ing the upper portions of the still. According to this law, a heavy 

 oil will yield the readier a light distillate, the higher the vapors have 

 to rise before leaving the still, because the reduction of temperature 

 in those higher portions of a retort, which are more remote from the 

 source of heat, acts upon the vapor of the oil in fine division, and 

 reduces their gravity more readily than the compact liquid oil. If 

 the heat is applied solely to the bottom of the still, while its sides and 

 top may be expose4 to a ciirreijt of cQol ^ir^ the reductipn of th§ teni- 



