Sentence:  Some of the oil-shops colour this [essence of anchovy] with bole armeniac, rose pink, Venice-red, &c.; but all these additions deteriorate the flavour of the anchovy, and the palate and stomach suffer for the gratification of the eye, which, in culinary concerns, will never be indulged by the sagacious "gourmand", at the expense of these two <WSD>"primum mobiles</WSD>" of his pursuits. 
Meaning of the word marked with <WSD> tags:  The person or thing that is the main impetus for some action; a driving force.