Why did still life paintings become so popular in the 18th century? what does the author state is the intention of the still life artist?

Although still life paintings were particularly abundant in the 17th century, they were also very popular in the 18th century, specifically among aristocrats, who acquired them as a sign of good taste. Advancing the century, they also began to be popular in the US among merchants, bankers and other professionals, thanks to painter Charles Willson Peale and his relatives.  

I am afraid I cannot fully answer the second question, since I don't know which author you are referring to, but still life artists often did not only attempt to depict the subject matter per se (a vase with flowers, a table with food, or a set of musical instruments) but also to send allegoric, symbolic and moral messages - a skull, an hourglass or a ripe yet juicy and sumptuous fruit could indicate life impermanence or disdain for material pleasures, for instance.        

i discarded the supernatant and kept the pellet in this part of the experiment because you have to return all unused chemicals into their original containers at the end of each labratory.

explanation:

d. the right to establish a one-house legislature