How does the Szilard-Einstein refrigerator design evaporate its refrigerant?

[In their 1927 patent](https://patentimages.storage.googleapis.com/53/e9/74/2cde176701fab8/US1781541.pdf), Leo Szilard and Albert Einstein use butane as a refrigerant in their non-compressive refrigerator design. In the patent the authors begin the description of the principle of operation by stating:



> 
> A suitable refrigerant, for instance butane, in liquid form is contained within evaporator 1. An inert gas, for instance ammonia, is in troduced into evaporator 1 through conduit
> 30 and distributor head 31. The refrigerant evaporates in the evaporator in the presence of the inert gas **due to the fact that the partial pressure of the refrigerant is reduced** thereby and the resulting gaseous mixture passes through conduit 5 to within condenser 6. *(Emphasis added)*
> 
> 
> 


**Why would adding gaseous ammonia to the butane-ammonia mixture result in additional butane evaporating?**


The quote refers to the parts of the diagram shown below. A full color-coded diagram of the refrigerator design can be found [here](https://en.wikipedia.org/wiki/Einstein_refrigerator#/media/File:Einstein_Refrigerator_pat1781541_clarified.jpg).
[![enter image description here](https://i.stack.imgur.com/npxWz.jpg)](https://i.stack.imgur.com/npxWz.jpg)


