請問這句this made it so much lighter in weight than the air without that it pulled hard to rise into the sky中的that是關係代名詞還是從屬句?it又代表什麼?謝謝?

that 在這裡是連接詞，引導「副詞子句」表示 so much lighter 的結果，是很普通的 so...that 句型。而 it 是指氣球。

整句最須注意的是別讓 without 迷惑了。假如將 without 當成介係詞，而把 without that 連成一體看，那就搞不懂 that 指什麼了。

這句出處是 120 年前出版的《綠野仙蹤》The Wonderful Wizard of Oz。在當年的語言裡， without 可以表示 outside。所以 the air without 要一起看，把 that 和 without 斷開來：

the air without = the air outside = 氣球外的空氣

所以整句意思是：

(The balloon ..., ) for the air within it was hot, and this made it SO MUCH LIGHTER in weight than the air without THAT it pulled hard to rise into the sky.
= (氣球‧‧‧，)  因為氣球裡的空氣很熱，這使得氣球重量上比外面的空氣輕了許多許多，於是氣球就猛然一扯，升到空中。

"so much lighter" than the air w/o "that"---
that=adv cl
=so much lighter
=as a result
==the sentence pattern
=so<------that
that it pulled hard
=/=猛然一扯揪
that it pulled hard (itself)
=猛然一扯揪
=20 marks for Tommy  Baby BB from "one" to me from "The Wonderful Wizard of Oz, published 120 yrs.ago."
-------------------------------------
it=balloon
w/o=outside.
-------------------------------------
=20 marks for Tommy Baby BB
from "one" to me  from "The wonderful wizard of Oz, published 120 years ago"
--------------------------------------

整句是The balloon was by this time tugging hard at the rope that held it to the ground, for the air within it was hot, and this made it so much lighter in weight than the air without that it pulled hard to rise into the sky