Sentence:  Have we got among our <WSD>primum mobiles</WSD> the force of such elastic vapours as be generated for the purpose out of nonelastic substances, and are not incondensible? A good magazine of vitr[iolic] acid and powder'd marble would not only save the expence of air in feeding fire, but generate heat. 
Meaning of the word marked with <WSD> tags:  The prime mover or (effects follow).