462 Trowbridge and Sheldon — Magnetism of 



more dangerous than pure error since the ballast of truth they 

 contain will enable the error to navigate some distance, while 

 the unfreighted error would capsize at once. 



The shell is in one sense the product of secretion from the 

 mantle, as the mammalian tooth is derived from the ectoderm 

 of the jaw or the skeleton from the periosteum and cartilages. 

 Both are that and much more. It would be as reasonable to 

 say that a steam boiler in process of construction is the product 

 of the boy inside who holds the rivet-heads, as to claim that the 

 shell has no more significance than is implied in the term 

 " secretion of the mantle.''' 



The original theoretic protoconch may have been so, but, as 

 soon as it came into being, its development was governed by 

 the physical forces impinging upon it from all sides and 

 through it influencing the growth and structure of the soft 

 parts beneath. The Gastropod shell is the result of the action 

 and reaction between the physical forces of the environment 

 and the evolutionary tendencies of the organic individual. In 

 the Pelecypod we have the mechanical stresses and reactions of 

 one valve upon the other added to the category of influences. 

 To some extent it is doubtless as true that the animal is 

 moulded by its shell as it is that the shell is shaped by the soft 

 parts of the animal. This results in that correlation of struc- 

 ture which has enabled students to, in the main, correctly 

 judge of the relations of mollusks by their shell- characters, when, 

 the latter were intelligently studied and properly appreciated. 



Art. LYI. — The Magnetism of Nickel and Tungsten 

 Alloys /* by Johjst Trowbridge and Samuel Sheldon. 



Introductory. 



The fact that different kinds of steel, alloyed in small pro- 

 portions with tungsten or wolfram, and magnetized to satu- 

 ration, increase in specific magnetism, f has long been known. 

 Whether the same effect would result from the use of nickel 

 alloyed with tungsten has never been investigated. This 

 paper has for its object a partial answer to the query. It was 

 instigated by Mr. Wharton, proprietor of the American Nickel 

 Works, whose chemist, Mr. Riddle, kindly prepared the alloys 

 which have been employed. These alloys were in two groups. 

 The first, received in November, 1888, consisted of three bars 

 of the same shape, one being of pure nickel and the other two 



* From the Proceedings of the American Academy of Arts and Sciences. 

 f Jour. Chem. Soc, 18G8, xxi, 284, says 300 per cent. 



